Fix typo in travis script.

.travis.yml

@@ -117,7 +117,7 @@ before_install:
   - |
     if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then
       # adding apt repos in travis is really fragile => retry a couple of times.
-      sudo apt-add-respository 'ppa:ubuntu-toolchain-r/test'
+      sudo apt-add-repository 'ppa:ubuntu-toolchain-r/test'
       for i in {1..10}; do sudo apt-get update && break; done
       for i in {1..10}; do sudo apt-get install gcc-8  $(if [ "$BACKEND" = cpp ]; then echo -n "g++-8"; fi ) && break ; done
       sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-8 60 --slave /usr/bin/g++ g++ /usr/bin/g++-8