Revert "Fixed issue with gemWorkDir not specified"

This reverts commit 84706976d123fce894a21c5d2c247b6dc7d66aca.
Revert "Added support for gemWorkDir so that GEM working dir for JRubyExec can be customised by user"

This reverts commit 0b39b1314a0289f4010f68786f296bc59fd5f8ac.

Conflicts:
	src/main/groovy/com/github/jrubygradle/JRubyExec.groovy
Force jar-dependencies and jbundler to not attempt to install java dependencies via `gem install`

Fixes #79