Fix uninitialized constant Capistrano::Version (NameError)

When using bundler 1.4.0.pre.2, a NameError occurs:

/gems/bundler-1.4.0.pre.2/lib/bundler/capistrano.rb:7:in `<top (required)>': uninitialized constant Capistrano::Version (NameError)

This fix explicitly requires the 'capistrano/version' file, and the error no longer exists.