Set TERM to 'dumb' when Gradle config is detected

There are following issues with #240 (dfe2bfc):

- Android builder does not export TERM=dumb (common parts are in jdk.rb,
  not jvm.rb)
- TERM=dumb would be exported for all JVM languages, even if not built
  with Gradle.
Merge pull request #241 from travis-ci/gh-2275-gradle-change

Set TERM to 'dumb' when Gradle config is detected