Clean up dependencies

The tests fail with rspec 3, so rspec has been set to ~> 2.99. Duplicates have
been removed.
Update rspec to the latest version
correctly use keyword arguments
specify bundle exec rspec and disable notifications in guardfile
Initialize guard as part of the specs

Without this call, an error message appears when running the tests about
Guard.state being nil.
Handle RSpec deprecation warnings

This includes using expect instead of should and removing obsolete config
settings.