Fix mac travis: check if libs existed before install (#47)

.travis.yml

@@ -48,7 +48,14 @@ matrix:
 install:
   # osx dependencies
   - if [ "${TRAVIS_OS_NAME}" == osx ]; then
-      brew install gflags zstd lz4 snappy xz;
+      to_install=( "gflags" "zstd" "lz4" "snappy" "xz" );
+      brew list > tmp;
+      for ins in ${to_install[*]}; do
+        grep -q "$ins" tmp;
+        if [ $? -ne 0 ]; then
+          brew install "$ins";
+        fi
+      done
     fi
   - git clone --depth=1 --branch=tikv-3.0 https://github.com/pingcap/rocksdb.git
   - export CTEST_OUTPUT_ON_FAILURE=1