Revert "Use rocksdb in the data test suite" (info below)

This reverts commit 49c49d818c5ecb87fb5bd6f6066850251f3c303e.

In order to use rocksdb we need g++ >= 4.7 (which has support for
C++0x11). It's too expensive to build g++ every time. Upgrading the
ubuntu distro used by travis seems to be coming soon, I'll shelf this
for now until travis-ci/travis-ci#2046 is resolved.