Plugin do not compute rake path correctly with custom gemsets

[hron84]

Referring to #4 it is currently not works correctly, because it uses the global gemset instead of the selected.
The problem described longer here: https://issues.jenkins-ci.org/browse/JENKINS-12786

I kindly ask to review this issue.

PS: Sorry for cross-posting, not clear for me where can I post issues for this plugin.

[gehel]

Same problem for me. Rake is run with the global gemset, not with the one specified in the parameter.