investigate renaming `uru .` to `uru auto`

[Jon]

• keep uru . as alias to uru auto
• as a separate issue, consult .ruby-version when given uru auto@gemset and investigate other ways to spell the horrifically ugly .@gemset
• Consider removing uru . support completely as uru auto and uru auto@gemset are cleaner and have the minimal cost of making auto a reserved uru word (prohibit users from tagging a ruby as auto)

[Jon]

Replace uru . with uru auto. Closes #58

This change breaks backward compatibility since all support for uru . has been removed. The change was made so that use of a .ruby-version file to automagically set the ruby version would be consistent regardless of whether one uses uru auto or uru auto@gemset.

→ <<cset eb0f1af8b51e>>