Easier testing with guard - sets rspec to a specific version

Adds `guard` (or more specifically [guard-spec](https://github.com/guard/guard-rspec)) to the `Gemfile` for easier and faster testing. Check the Contribution guidelines for a more detailed explanation.

Also ignores the `Gemfile.lock` file and sets `rspec` to a specific version in the `Gemfile`. splitjoin is using rspec syntax that has been deprecated in the recent 3.x releases of `rspec` - using such a version will throw lots of deprecation warnings. This PR defines to use one of the latest 2.14.x releases of rspec.