GenerateGradleRb not using its Configuration to determine the gemInstallDir

[rtyler]

generateGradleRb.configuration project.configurations.mySpecialConf

Will still generate a gradle.rb with the GEM_PATH and GEM_HOME set to build/gems instead of the directory that JRubyPrepare will use for mySpecialConf

[rtyler]

Somewhat related (in my head at least) to #237

[jamescway]

Would it make sense to set this.gemInstallDir = project.tasks.JrubyPrepare.outputDir when the GenerateGradleRb::configuration(Object config) gets called with argument mySpecialConf?

[rtyler]

@jamescway something like that would need to happen and something to address #237. Without #237 right now there isn't a jrubyPrepare task for non-default configuraitons, so mySpecialConf wouldn't actually have gems installed without it.