fixes compatibility with guard-bundler gem

Changes instances of Bundler to ::Bundler to prevent it from attempting
to reference Guard::Bundler.

When attempting to run with both gems installed, you would receive the following error:

    Guard::RSpec failed to achieve its <run_all>, exception was:
    > [#F722A29A3B60] NoMethodError: undefined method `with_clean_env' for Guard::Bundler:Class