[TASK] Clean up the Gem management.

Gems should store all their dependencies in the gemspec file, not in the Gemfile.

In addition, they should have very loose depedencies so that they can work
for a long time.

This change includes the following changes:

* Delete Gemfile.lock.
* Add Gemfile.lock to .gitignore.
* Move the development dependencies from the Gemfile to the gemspec file.
* Drop the version requirement for sass.
* Drop the version requirement for aruba.