[Fix #437] Always set $TMPDIR
Do not substitute /tmp since $TMPDIR is always set
Add the RubyGems bin directory to PATH on other Unix systems

Signed-off-by: Sorin Ionescu <sorin.ionescu@gmail.com>
[Fix #436] Remove Bombich rsync references
Merge branch 'master' of git://github.com/sorin-ionescu/prezto