What is the basic workflow for gems with Bundler and Heroku?

Question

I have a Gemfile in the root of the app directory where I specify gems with versionnumber either like this:

gem 'rails', '3.2.14'

or in a environment block (where applicable) like this:

group :production do
  gem 'pg', '0.12.2'
end

When I add a new gem I run the command

bundle install

and when I want to update all gems I run the comand

bundle update

When I deploy my app to Heroku I run

git push heroku master

Is there anything I'm missing here in the workflow, or could be improved? Because right now I'm getting an error on Heroku (after having updated all gems), stating the following

can't activate bcrypt-ruby (~> 3.0.0, runtime), already activated bcrypt-ruby-3.1.2. Make sure all dependencies are added to Gemfile. (Gem::LoadError)

And I cannot figure out what I'm doing wrong. The application works perfectly fine locally. Clarifications on what logical missteps I'm doing would be greatly appreciated.

Update

The Gemfile.lock:

GEM
  remote: https://rubygems.org/
  specs:
    actionmailer (3.2.14)
      actionpack (= 3.2.14)
      mail (~> 2.5.4)
    actionpack (3.2.14)
      activemodel (= 3.2.14)
      activesupport (= 3.2.14)
      builder (~> 3.0.0)
      erubis (~> 2.7.0)
      journey (~> 1.0.4)
      rack (~> 1.4.5)
      rack-cache (~> 1.2)
      rack-test (~> 0.6.1)
      sprockets (~> 2.2.1)
    activemodel (3.2.14)
      activesupport (= 3.2.14)
      builder (~> 3.0.0)
    activerecord (3.2.14)
      activemodel (= 3.2.14)
      activesupport (= 3.2.14)
      arel (~> 3.0.2)
      tzinfo (~> 0.3.29)
    activeresource (3.2.14)
      activemodel (= 3.2.14)
      activesupport (= 3.2.14)
    activesupport (3.2.14)
      i18n (~> 0.6, >= 0.6.4)
      multi_json (~> 1.0)
    arel (3.0.2)
    bcrypt-ruby (3.1.2)
    builder (3.0.4)
    daemons (1.1.9)
    erubis (2.7.0)
    eventmachine (1.0.3)
    execjs (2.0.1)
    hike (1.2.3)
    hirb (0.7.1)
    httpclient (2.3.4.1)
    i18n (0.6.5)
    impressionist (1.4.7)
      httpclient (~> 2.2)
      nokogiri (~> 1.6.0)
    journey (1.0.4)
    jquery-rails (2.2.1)
      railties (>= 3.0, < 5.0)
      thor (>= 0.14, < 2.0)
    jquery-ui-rails (4.0.4)
      jquery-rails
      railties (>= 3.1.0)
    json (1.8.0)
    mail (2.5.4)
      mime-types (~> 1.16)
      treetop (~> 1.4.8)
    mime-types (1.25)
    mini_portile (0.5.1)
    multi_json (1.7.9)
    nokogiri (1.6.0)
      mini_portile (~> 0.5.0)
    pg (0.12.2)
    polyglot (0.3.3)
    quiet_assets (1.0.2)
      railties (>= 3.1, < 5.0)
    rack (1.4.5)
    rack-cache (1.2)
      rack (>= 0.4)
    rack-ssl (1.3.3)
      rack
    rack-test (0.6.2)
      rack (>= 1.0)
    rails (3.2.14)
      actionmailer (= 3.2.14)
      actionpack (= 3.2.14)
      activerecord (= 3.2.14)
      activeresource (= 3.2.14)
      activesupport (= 3.2.14)
      bundler (~> 1.0)
      railties (= 3.2.14)
    railties (3.2.14)
      actionpack (= 3.2.14)
      activesupport (= 3.2.14)
      rack-ssl (~> 1.3.2)
      rake (>= 0.8.7)
      rdoc (~> 3.4)
      thor (>= 0.14.6, < 2.0)
    rake (10.1.0)
    rdoc (3.12.2)
      json (~> 1.4)
    sass (3.2.10)
    sass-rails (3.2.6)
      railties (~> 3.2.0)
      sass (>= 3.1.10)
      tilt (~> 1.3)
    sprockets (2.2.2)
      hike (~> 1.2)
      multi_json (~> 1.0)
      rack (~> 1.0)
      tilt (~> 1.1, != 1.3.0)
    sqlite3 (1.3.7)
    thin (1.5.1)
      daemons (>= 1.0.9)
      eventmachine (>= 0.12.6)
      rack (>= 1.0.0)
    thor (0.18.1)
    tilt (1.4.1)
    treetop (1.4.15)
      polyglot
      polyglot (>= 0.3.1)
    tzinfo (0.3.37)
    uglifier (2.2.1)
      execjs (>= 0.3.0)
      multi_json (~> 1.0, >= 1.0.2)
    webrick (1.3.1)

PLATFORMS
  ruby

DEPENDENCIES
  bcrypt-ruby (= 3.1.2)
  hirb
  impressionist
  jquery-rails (= 2.2.1)
  jquery-ui-rails
  pg (= 0.12.2)
  quiet_assets
  rails (= 3.2.14)
  sass-rails (~> 3.2.6)
  sqlite3 (= 1.3.7)
  thin
  uglifier (>= 1.3.0)
  webrick (~> 1.3.1)