Add format_version=5 to db_crashtest (#6102)

Summary:
format_version=5 enables new Bloom filter. Using 2/5
probability for "latest and greatest" rather than naive 1/4.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6102

Test Plan: start 'make blackbox_crash_test'

Differential Revision: D18735685

Pulled By: pdillinger

fbshipit-source-id: e81529c8a3f53560d246086ee5f92ee7d79a2eab

tools/db_crashtest.py

@@ -66,7 +66,7 @@ default_params = {
     "verify_checksum": 1,
     "write_buffer_size": 4 * 1024 * 1024,
     "writepercent": 35,
-    "format_version": lambda: random.randint(2, 4),
+    "format_version": lambda: random.choice([2, 3, 4, 5, 5]),
     "index_block_restart_interval": lambda: random.choice(range(1, 16)),
     "use_multiget" : lambda: random.randint(0, 1),
     "periodic_compaction_seconds" :