Warn about env SPEC_OPTS causing notification problems.

I just setup guard/rspec/spork/growl on OSX. Everything was working except for growl notifications after rspec finished running. I finally figured out that I had set SPEC_OPTS in my bash profile and that this was overriding the options sent to spork.

Specifically it was overriding the format to use my SPEC_OPTS supplied format instead of Guard::RSpec::Formatter. This is a problem since that class is the one responsible for sending growl notifications.

This patch adds functionality to produce a warning if SPEC_OPTS is present.

This pull request is in reference to https://github.com/guard/guard-rspec/issues/155

![warn-if-spec-opts-present](https://f.cloud.github.com/assets/16705/168860/cce3d5a6-7a0b-11e2-95d6-723282d25cb1.png)