[~] fix plugin pj

https://github.com/robbyrussell/oh-my-zsh/commit/bce74975d055529cbd186782e2fd99e6da840460
Switch back to oh-my-zsh.hide-dirty setting again

Commit 81004dfaba509ff62a13ba303ab941938d619326 reverted the change
in 9b811fb625c03c30a766191cdf65a1c7c1fd96b2 when editing the merge
conflict from #2928.

This commit fixes that so that we don't make the same mistake again.

First seen in http://git.io/Cdaj5Q
Merge pull request #3323 from alfss/fix-pj

[~] fix plugin pj
Merge pull request #3333 from mcornella/fix-git-hide-dirty

Switch back to oh-my-zsh.hide-dirty setting again
Add warning if npm is not in $PATH