Darwin should be compatible with java 7

[tmnd1991]

Scenario summary

As of 1.0.9 Darwin supports only java 8 onwards.

Problem statement

Some old projects are still based on java 7. Scala 2.12 dropped the support for java 7.

Proposed solution

Compile scala 2.10 and scala 2.11 artifacts of Darwin with java 7 compatibility and scala 2.12 Darwin artifacts with java 8 compatibility.

[tmnd1991]

Java 7 has been chosen over version 6 because HBase requires Java 7.

The patch has been tested building the artifacts locally and checking the class file:

Bytes 6 and 7 indicate the java major version:

scala version	java major hex	java major
2.10	0x33	7
2.11	0x33	7
2.12	0x34	8

Before the changes the output of the commands were: