Command not respecting bundle

When running guard-rspec on a system with multiple versions of rspec in place guard-rspec uses the latest version of rspec on the system rather than the one specified in the gem file. I've traced it down to [lib/guard/rspec/runner.rb:40](https://github.com/guard/guard-rspec/blob/master/lib/guard/rspec/runner.rb#L40) where it explicitly blows away the existing Bundler environment. This leads to the wrong version of rspec getting loaded preventing any specs from being run.

My project's `Gemfile` and `Guardfile` along with a console log of this occurring are contained in [this gist](https://gist.github.com/tpickett66/fc0e8218bc02f0d438b1).

I'm going to work on a patch to correct this behavior but would love some insight as to the original design decision that was made to introduce this behavior.
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