Add option to use bundle exec

[sanelson2000]

This PR adds a checkbox to the task to use bundle exec. It follows all of the same ruby path lookup rules as the actual rake command and will result in a command that looks something like this:

/rubyhome/bin/bundle(.bat) exec rake task

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plugins » rake-plugin #7 SUCCESS
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plugins » rake-plugin #11 SUCCESS
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[sanelson2000]

I updated the PR using spaces instead of tabs as requested by @olivierdagenais. NOTE: config.jelly is littered with a bunch of tab characters not related to my changes. I only changed the lines that I edited to avoid confusion on the commit.

[olivierdagenais]

OK, please move the if (bundleExec) block and delete the unused methods and we're good to go.

[cloudbees-pull-request-builder]

plugins » rake-plugin #12 SUCCESS
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[sanelson2000]

OK. Done.

[olivierdagenais]

Thanks for the pull request and your patience! :)

[sanelson2000]

No problem. Thank you!