Use '.' instead of 'source'.

This enhances compatibility with other shell like dash(1). The command
'eval "$(rbenv init -)"' may end up in a .zshenv file which in turns is called
by session initialization script. This way programs started by the window
manager (specially IDE such as Emacs) inherits from the right environment
and thus can use the ruby from rbenv. These initialization script may not use
bash(1).