Upgrade plovr to use v20130912 of closure-library and closure-compiler.

Hello @nicks, 

Please review the following commits I made in branch 'nick-merge'.

b0bee302b1e1e0392165cafc0e30437e866dadbb (2013-09-12 15:05:35 -0400)
Upgrade plovr to use v20130912 of closure-library and closure-compiler.


R=@nicks