build: Set heap size for main test bucket

Travis container based build was having intermittent failures running
the biz.aQute.bndlib.test:test target:

FAILURE: Build failed with an exception.

* What went wrong:

Execution failed for task ':biz.aQute.bndlib.tests:test'.

> Process 'Gradle Test Executor 6' finished with non-zero exit value 137

Signed-off-by: BJ Hargrave <bj@bjhargrave.com>
Merge pull request #753 from bjhargrave/gradle-stacktrace

build: Set heap size for main test bucket