added correct / missing license MPL headers

Signed-off-by: Michael Vorburger <mike@vorburger.ch>
license-gradle-plugin updated + license header presence verified for each pull request via travis_build.sh now
Updated Contributing.md with clarification that license header presence is verified on pull requests
Merge pull request #1053 from vorburger/TravisLicenseTest

Travis will now test presence of MPL license headers for each pull request it builds