Proposed changes to the handling of default repositories

FIXES #13, #17.

Key updates:
* `rubygemRelease()` is a repository name that can be used like `jcenter()`
* Default repositories can be turned off `jruby.defaultRepositories`
* Default repositories are only loaded in the `afterEvaluate` phase.

This PR also addresses some ordering issues between `test` and `integrationTest` tasks. THe latter should not run before the former.
What's this change for?