Update to installation instructions in README.md

When following the installation instructions, running the command "sudo gem install capistrano -v 2.15 capistrano-ext colored" from line 41 causes the following error:

"ERROR:  Can't use --version w/ multiple gems. Use name:ver instead."

The proposed code fixes this and allows the install to be completed.
Reverted changes by chrome plugin.
Merge pull request #1 from jameswlane/patch-2

Reverted changes by chrome plugin.
Merge pull request #104 from TaishiZiyi/patch-1

Installation Documentation Fix