Multi-stage CI to compile Java 8 code with Java 7 target

[hhandoko]

Context

Kotlin 1.1+ and KotlinTest 2.x requires JDK 8 to compile, but it can set to target Java 7 bytecode. Splitting Travis CI pipeline into stages, it will allow the library to be compiled with an up-to-date version of Kotlin whilst maintaining Java 7 compatibility.

Related issues: #35, #58, #59

Refer to Travis CI Build Stages documentation for more information.

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Definition of Done

Compile the code using Java 8 and run integration test in a separate stage using Java 7.