isolate gem environment if gemPath is null

- unset GEM_PATH and GEM_HOME
- set GEM_HOME to first path-segment of GEM_PATH if gemPath is provided
- set environment directly on JRubyInstanceConfig instead of using evalScriptlet
also unset GEM_PATH & GEM_HOME if GEM_PATH is an empty string
Merge pull request #187 from mojavelinux/isolate-env

isolate gem environment if gemPath is null or empty string