Specify all dependiencies in gemspec and upgrade some gems

* Fix compatibility with sequel 4.0.0
* Upgrade rspec and fix deprecation warnings
* Manage all dependencies through gemspec
Remove refernces to rdoc
Merge pull request #68 from TSFoster/autocomplete

Autocomplete
Merge branch 'master' into autocomplete
Move autocomplete files to shell specific directories
Add instructions for installing completions to README