fix plovr concurrency problems (upstream as https://github.com/bolinfest/plovr/pull/25)

Hello @nicks, 

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

13158fe682927a0efab7f38a0473f21d7a75dc3b (2014-08-21 23:41:12 -0400)
bump plovr version

e3fae777ebd435d64bed169f5cd2ae92afb44552 (2014-08-21 17:19:55 -0400)
Use multi-threaded guava caches instead of unsynchronized maps

R=@nicks