Skip ldb test in Travis

Summary:
Travis now is building for ldb tests. Disable for now to unblock other tests while we are investigating.
Closes https://github.com/facebook/rocksdb/pull/1546

Differential Revision: D4209404

Pulled By: siying

fbshipit-source-id: 47edd97

Makefile

@@ -709,7 +709,8 @@ ifeq ($(filter -DROCKSDB_LITE,$(OPT)),)
 	sh tools/rocksdb_dump_test.sh
 endif
 
-check_some: $(SUBSET) ldb_tests
+# TODO add ldb_tests
+check_some: $(SUBSET)
 	for t in $(SUBSET); do echo "===== Running $$t"; ./$$t || exit 1; done
 
 .PHONY: ldb_tests