Replace stub gems with the Real McCoy.

Due to a rubygems security fix which arrived in JRuby 9.1.15.0, the empty stub
gems we were using in our integration tests would no longer work properly,
causing the error below:

    installing into parent path
    /home/tyler/source/github/jruby-gradle/jruby-gradle-plugin/jruby-gradle-base-plugin/build/tmp/integrationTest/root/jpgis/rack-resolve/build/gems/gems/sinatra-1.4.5 of
    /home/tyler/source/github/jruby-gradle/jruby-gradle-plugin/jruby-gradle-base-plugin/build/tmp/integrationTest/root/jpgis/rack-resolve/build/gems/gems/sinatra-1.4.5 is not allowed`

By adding the real gem files, the tests work fine again 😸

Many 👏 for @ysb33r whose detective work helped uncover the empty gems in
relation to the test failure

Author: rtyler