Fix nvm install script

.travis.yml

@@ -43,7 +43,7 @@ before_install:
   - echo "deb http://dl.yarnpkg.com/debian/ stable main" | sudo tee /etc/apt/sources.list.d/yarn.list
   - sudo apt-get update -qq
   - sudo apt-get install -y -qq yarn=0.24.6-1
-  - rm -rf ~/.nvm && git clone https://github.com/creationix/nvm.git ~/.nvm && (cd ~/.nvm && git checkout \`git describe --abbrev=0 --tags\`) && source ~/.nvm/nvm.sh && nvm install 8.0.0" || exit $?
+  - rm -rf ~/.nvm && git clone https://github.com/creationix/nvm.git ~/.nvm && (cd ~/.nvm && git checkout $(git describe --abbrev=0 --tags)) && source ~/.nvm/nvm.sh && nvm install 8.0
 
 install:
   - composer install -n