Fix travis builds

Summary:
Lots of travis builds are failing because on EnvPosixTest.RandomAccessUniqueID: https://travis-ci.org/facebook/rocksdb/builds/34400833

This is the result of their environment and not because of RocksDB's bug.

Also note that RocksDB works correctly even though UniqueID feature is not present in the system (as it's the case with os x)

Test Plan:
OPT=-DTRAVIS make env_test && ./env_test
Observed that offending tests are not being run

Reviewers: sdong, yhchiang, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D22803