Add the correct drush version

.travis.yml

@@ -16,7 +16,7 @@ env:
   - BASE_URL="http://127.0.0.1:8080"
 
 install:
-  - composer global require drush/drush:8
+  - composer global require "drush/drush:~8"
 
 before_script:
   - docker pull statonlab/tripal2