Fix CONTRIBUTING.md
Merge pull request #68 from maurizi/patch-1

Fix CONTRIBUTING.md
Set the executable bit for scripts in the repository

This was actually done on Windows using

$ git update-index --chmod=+x <file>

and allows us to simplify the build instructions a bit.
Merge pull request #71 from sschuberth/master

Set the executable bit for scripts in the repository
Update to Gradle 2.1, Android plugin 0.14.0 and coveralls plugin 2.0.0