Install GoogleTest with homebrew in macOS (#20)

### Motivation

Install `googletest` with homebrew in macOS is more convenient than manual compilation.

### Modifications

Replace manual compilation with `brew install googletest`

Author: maxsxu

README.md

@@ -158,14 +158,8 @@ export OPENSSL_ROOT_DIR=/usr/local/opt/openssl/
 brew install protobuf boost boost-python log4cxx jsoncpp
 // If you are using python3, you need to install boost-python3
 
-# For gtest
-cd $HOME
-git clone https://github.com/google/googletest.git
-cd googletest
-git checkout release-1.12.1
-cmake .
-make install
-# Refer gtest documentation in case you get stuck somewhere
+# For GoogleTest
+brew install googletest
 ```
 
 #### Compile Pulsar client library: