travis: add CMake compilation

Summary:
siying
Closes https://github.com/facebook/rocksdb/pull/2233

Differential Revision: D4987113

Pulled By: siying

fbshipit-source-id: f07ecd3f7d9f5366a2b1665ce28ba10d74405557

.travis.yml

@@ -29,6 +29,7 @@ env:
   - JOB_NAME=lite_build
   # Build examples
   - JOB_NAME=examples
+  - JOB_NAME=cmake
 
 matrix:
   exclude:
@@ -56,6 +57,7 @@ script:
   - if [[ "${JOB_NAME}" == 'java_test' ]]; then OPT=-DTRAVIS V=1 make clean jclean rocksdbjava jtest; fi
   - if [[ "${JOB_NAME}" == 'lite_build' ]]; then OPT="-DTRAVIS -DROCKSDB_LITE" V=1 make -j4 static_lib; fi
   - if [[ "${JOB_NAME}" == 'examples' ]]; then OPT=-DTRAVIS V=1 make -j4 static_lib; cd examples; make -j4; fi
+  - if [[ "${JOB_NAME}" == 'cmake' ]]; then mkdir build && cd build && cmake .. && make -j4 rocksdb; fi
 notifications:
     email:
       - leveldb@fb.com