Fix a bug that was causing rake to overwrite SHELL settings

The prezto install procedure now checks to make sure the user
doesn't have an alternative version of zsh installed before changing
the current shell preference with "chsh".
This may be the case when an user is already using zsh but on a
different version from the one shipped with OS X

Fixes #264
Merge pull request #265 from duhanebel/zsh_install_check

Fix a bug that was causing rake to overwrite SHELL settings