Installation Documentation Fix

Fixed an issue in the installation documentation where the user is instructed to run the following command:

"sudo gem install capistrano -v 2.15 capistrano-ext colored"

This results in an error instructing the user that the --version syntax is invalid when specifying multiple gems.  The command should instead be:

"sudo gem install capistrano:2.15 capistrano-ext colored"

Running the command with this syntax works without error.