Fix Gem::Specification#add_development_dependency arguments.

This method doesn't accept an array. Or at least it doesn't with the version
of RubyGems that I'm using.

The following gems are not essential for running the tests, so I wouldn't
include them in the gemspec:

- guard
- guard-rspec
- rb-fsevent
Add a Gemfile to make development easier.

This simply includes all the dependencies specified in the gemspec.

Also git ignore `Gemfile.lock` which doesn't need to be stored in the repo.
Fix brittle environment-specific test.

My default shell is `/bin/zsh` and so this test was failing.

The example from the `String#sub` documentation is just demonstrating the use
of a `Hash` as the second argument, so there is no need for it to actually be
`ENV`.
Merge pull request #2 from floehopper/small-tweaks

Small tweaks