"ccache -C" in Travis

Summary:
This is to work around the problem of build error:

util/threadpool_imp.o: file not recognized: File truncated

Just to make the build go through. We should remove it later if we find the real long-term solution.
Closes https://github.com/facebook/rocksdb/pull/2657

Differential Revision: D5511034

Pulled By: siying

fbshipit-source-id: 229f024bd78ee96799017d4a89be74253058ec30

.travis.yml

@@ -57,7 +57,7 @@ before_script:
 
 script:
   - ${CXX} --version
-  - if [ "${TEST_GROUP}" == 'platform_dependent' ]; then OPT=-DTRAVIS V=1 make -j4 all; OPT=-DTRAVIS V=1 ROCKSDBTESTS_END=db_block_cache_test make -j4 check_some; fi
+  - if [ "${TEST_GROUP}" == 'platform_dependent' ]; then ccache -C && OPT=-DTRAVIS V=1 make -j4 all && OPT=-DTRAVIS V=1 ROCKSDBTESTS_END=db_block_cache_test make -j4 check_some; fi
   - if [ "${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 [ "${TEST_GROUP}" == '2' ]; then OPT=-DTRAVIS V=1 ROCKSDBTESTS_START=comparator_db_test make -j4 check_some; fi
   - if [ "${JOB_NAME}" == 'java_test' ]; then OPT=-DTRAVIS V=1 make clean jclean && make rocksdbjava jtest; fi