prepareJRubyJar doesn't cache results and reinstalls gems every single run

[rtyler]

Because we're now installing gems into build/dirinfo/jrubyJar by default, this line ensures that the directory is completely cleaned every time the JRubyDirInfo object is instantiated.

This is obviously problematic. And this should be backported to v1.0.4

Originally identified as jruby-gradle/jruby-gradle-storm-plugin#22

[mkristian]

this current implementation is problematic due to following issues:

• collecting the dir data in memory is not possible since this hook

  jruby-gradle-plugin/jruby-gradle-jar-plugin/src/main/groovy/com/github/jrubygradle/jar/JRubyJar.groovy

  Line 245 in 29bcf7a

  task.getSource().visit {

  is just a visitor and there is no "end of visit" callback
• the relative path of the visitor is the source path and the not the target path which can be different

+1 for a better implementation

[rtyler]

I think I'll remove the deleteDir from now to prevent this regression and open another enhancement request to more cleanly stage and generate the .jrubydir information when we create the jar artifact instead of the stage directory