Use rubocop in .travis.yml

Author: isuruf

.travis.yml

@@ -26,7 +26,7 @@ matrix:
   include:
     - os: linux
       rvm: 2.0
-      env: BUILD_TYPE="Debug" WITH_BFD="yes" WITH_MPC="yes"
+      env: BUILD_TYPE="Debug" WITH_BFD="yes" WITH_MPC="yes" RUBOCOP="yes"
     - os: linux
       rvm: 2.3.0
       env: BUILD_TYPE="Debug" WITH_BFD="yes" WITH_MPC="yes"
@@ -66,6 +66,8 @@ script:
   - export CFLAGS="-Werror"
   # Install ruby gem
   - gem install symengine-0.1.0.gem --install-dir $HOME --verbose -- -DSymEngine_DIR=$our_install_dir
+  # Check for code style using rubocop
+  - if [[ "${RUBOCOP}" == "yes" ]]; then rubocop; fi
 
   # Test ruby gem
   - gem install gem-path --no-ri --no-rdoc