error-prone version not compatible with Java 1.6

[davidzchen]

The error-prone version currently does not work with Java 1.6:

[gaggarwa@gaggarwa-ld1 azkaban]$ ./gradlew distTar
FAILURE: Build failed with an exception.
* Where:
Build file '/home/gaggarwa/bug/azkaban/build.gradle' line: 39
* What went wrong:
A problem occurred evaluating root project 'azkaban'.
> java.lang.UnsupportedClassVersionError: net/ltgt/gradle/errorprone/ErrorPronePlugin : Unsupported major.minor version 51.0
* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.
BUILD FAILED
Total time: 1.771 secs

I'll see if I can conditionally use a different version of error-prone when building with Java 1.6.

[davidzchen]

As discussed with @hluu offline, Azkaban 3 will no longer support Java 1.6.