Upgrade to LevelDB 1.14.0 to fix LevelDB bugs.

This tentatively fixes https://github.com/prometheus/prometheus/issues/368 due
to an upstream bugfix in snapshotted LevelDB iterator handling, which got fixed
in LevelDB 1.14.0:

https://code.google.com/p/leveldb/issues/detail?id=200

Change-Id: Ib0cc67b7d3dc33913a1c16736eff32ef702c63bf