Update .submodules to point to pingcap/rocksdb tikv-3.0 branch (#284)

Update .gitmodules to point to pingcap/rocksdb tikv-3.0 branch. Also update rocksdb to include the following changes:

75133b1b6 2019-03-26 yiwu@pingcap.com Fix SstFileReader not able to open ingested file (#5097)
7ca9eb754 2019-03-26 yiwu@pingcap.com Fix BlockBasedTableIterator construction missing index_key_is_full parameter
Signed-off-by: Yi Wu <yiwu@pingcap.com>

.gitmodules

 [submodule "rocksdb"]
 	path = librocksdb_sys/rocksdb
 	url = https://github.com/pingcap/rocksdb.git
-	branch = titan-5.15
+	branch = tikv-3.0

rocksdb @ ee1605a6

-Subproject commit 3db17e0be16f48bd1483424eefe0eaaaa7077260
+Subproject commit ee1605a6ef88cd65e34a84a784fb3afeef606611