Update README.md and CONTRIBUTING.md

Signed-off-by: Steven Downer <grinch@outlook.com>
Remove part of Contributing block in README.md
Update for Gradle 2.0 compatibility

Signed-off-by: Steven Downer <grinch@outlook.com>
Update .travis.yml to use default Gradle on Travis

Signed-off-by: Steven Downer <grinch@outlook.com>