Fix typo in travis script.

.travis.yml

@@ -112,7 +112,7 @@ before_install:
   - |
     if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
       sudo apt-get update
-      sudo apt-get install gcc-8  $(if [ "$BACKEND" = cpp ]; then echo -n "g++-8" )
+      sudo apt-get install gcc-8  $(if [ "$BACKEND" = cpp ]; then echo -n "g++-8"; fi )
       sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 60
       sudo update-alternatives --set gcc /usr/bin/gcc-8
       export CC=gcc-8