The Jack toolchain is deprecated. Jack and Jill was an experimental android build tool chain intended to add Java 8 features to Android. These tools created dex files directly from java source code, thus skipping dexing and proguarding steps.
From: http://tools.android.com/tech-docs/jackandjill
The Jack toolchain is deprecated, as per this announcement. However, you may continue to use it to enable Java 8 language features until the replacement becomes available.
Jack is an experimental Android toolchain that comprises a compiler from Java programming language source to the Android dex file format. Jack has its own .jack library format and provides most tool chain features as part of a single tool: repackaging, shrinking, obfuscation and multidex. Jill is a tool that translates existing .jar files to the .jack library format.
More information can be found on the official tech-docs.