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

This reverts commit 49c49d818c.

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.

integration/test_config_single.toml

@@ -83,7 +83,7 @@ dir = "/tmp/influxdb/development/db"
 write-buffer-size = 10000
 
 # the engine to use for new shards, old shards will continue to use the same engine
-default-engine = "rocksdb"
+default-engine = "leveldb"
 
 # The default setting on this is 0, which means unlimited. Set this to something if you want to
 # limit the max number of open files. max-open-files is per shard so this * that will be max.