Enable to switch to command mode by ";" in Vim

Map existent ";" to ":". So ";" and ":" are swapped.
Create .gemrc
Add --no-ri and --no-rdoc specifications
Modify from "--no-ri --no-rdoc" to "--no-document"

Because "--no-ri" and "--no-rdoc" are decprecated.

ref.
  http://guides.rubygems.org/command-reference/#gem-install
  http://qiita.com/kei_q/items/d13235157fcfc435489d