Alias ack to ack-grep only when applicable [zsh]

On Mac OS X `ack` is `ack` if installed via homebrew.
Use libtcmalloc_minimal when on OS X [zsh]
Remove Ruby GC env variables not present in MRI

And document better the other ones.
Use colordiff only when available [zsh]
Tweak Ruby GC env variables

Switch to default heap slots growth factor (should not be worse)
and decrease malloc limit to 150MB (which should be enough).
Add Ruby 2.1.0 GC settings

Not yet sure if they help or make things worse. Time will tell.