Latest master branch does not build successfully anymore.

[robinverduijn]

The latest master branch fails to build successfully:

• There is a failure trying to use JDK 8 at https://travis-ci.org/eriwen/gradle-css-plugin/builds/573584217
• The build is trying to resolve unresolvable Gradle dependencies at https://travis-ci.org/eriwen/gradle-css-plugin/builds/569832012

I have included fixes for both of these in my pull request at #53. The PR was originally intended to fix a Gradle deprecation warning but required the additional fixes so that it could pass its status checks.