Add long description to reset

To make it clear that you can optionally specify a level name
when calling `githug reset`
Add ruby 2.1 to travis config

So that githug is tested against this version of ruby too
Merge pull request #171 from mocoso/add-ruby-2-1

Add ruby 2.1 to travis config
Merge pull request #170 from mocoso/add-long-description-to-reset

Add long description to reset