Gradle Dependency Conflict, Version for app 26.1.0 and test app 27.1.1

Question

I am working on an android app having the navigation drawer. All things are working when When I got error of Floating action bar not found.

I have searched about that on stack overflow Link 1

and I found solution to add this dependency in the code implementation 'com.android.support:support-v4:26.1.0'

After I added I found another error of conflict between dependencies.

the Links I have searched to resolve my problems Link 2, Link 3, Link 4

Here is my app build.gradle Dependencies

dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'com.android.support:appcompat-v7:26.1.0'
implementation 'com.android.support:support-v4:26.1.0'
implementation 'com.android.support:cardview-v7:26.1.0'
implementation 'com.android.support.constraint:constraint-layout:1.1.2'
implementation 'com.google.firebase:firebase-auth:11.0.4'
implementation 'com.google.firebase:firebase-database:11.0.4'
implementation 'com.android.support:design:26.1.0'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'com.android.support.test:runner:1.0.2'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
}

android {
compileSdkVersion 26
defaultConfig {
    applicationId "com.example.usmanali.medicineinvent"
    minSdkVersion 16
    targetSdkVersion 26
    versionCode 1
    versionName "1.0"
    testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}
}

The other gradle file

    buildscript {
    repositories {
        google()
        jcenter()
    }
dependencies {
    classpath 'com.android.tools.build:gradle:3.0.1'
    // NOTE: Do not place your application dependencies here; they belong
    // in the individual module build.gradle files
    classpath 'com.google.gms:google-services:3.1.0'
}
}