Use more platform compatible text replacement command

In Mac OSX, sed(1) is BSD origin, so `-e` option is different to Unix's sed(1).

    $ mkdir /tmp/sed-incompatible; cd /tmp/sed-incompatible
    $ bundle init
    $ echo "ruby '1.9.3'" >> Gemfile
    $ sed -i -e 's/^ruby/#ruby/' Gemfile
    $ ls
    Gemfile         Gemfile-e                 #=> unexpected file `Gemfile-e` is created!

This commit changes replacement command to use ruby(1) as a platform compatible sed :)