fix travis failure for ccache command not found

Summary:
Travis don't have ccache installed on Mac. Only run the ccache command when it exists.
https://docs.travis-ci.com/user/caching/#ccache-cache
Closes https://github.com/facebook/rocksdb/pull/2990

Differential Revision: D6028837

Pulled By: yiwu-arbug

fbshipit-source-id: 1e2d1c7f37be2c73773258c1fd5f24eebe7a06c6

.travis.yml

@@ -60,7 +60,8 @@ before_script:
 
 script:
   - ${CXX} --version
-  - if [ "${TEST_GROUP}" == 'platform_dependent' ]; then ccache -C && OPT=-DTRAVIS V=1 ROCKSDBTESTS_END=db_block_cache_test make -j4 all_but_some_tests check_some; fi
+  - if [ `command -v ccache` ]; then ccache -C; fi
+  - if [ "${TEST_GROUP}" == 'platform_dependent' ]; then OPT=-DTRAVIS V=1 ROCKSDBTESTS_END=db_block_cache_test make -j4 all_but_some_tests check_some; fi
   - if [ "${TEST_GROUP}" == '1' ]; then OPT=-DTRAVIS V=1 ROCKSDBTESTS_START=db_block_cache_test ROCKSDBTESTS_END=comparator_db_test make -j4 check_some; fi
   - if [ "${TEST_GROUP}" == '2' ]; then OPT=-DTRAVIS V=1 ROCKSDBTESTS_START=comparator_db_test ROCKSDBTESTS_END=write_prepared_transaction_test make -j4 check_some; fi
   - if [ "${TEST_GROUP}" == '3' ]; then OPT=-DTRAVIS V=1 ROCKSDBTESTS_START=write_prepared_transaction_test make -j4 check_some; fi