.travis.yml: fix quoting

Author: df7cb

.travis.yml

@@ -14,7 +14,7 @@ sudo: required
 
 before_install:
   # apt.postgresql.org is already configured, we just need to add devel
-  - [ "$DIST" ] && sudo sed -i -e "s/pgdg.*/pgdg-testing main $PG_SUPPORTED_VERSIONS/" /etc/apt/sources.list.d/pgdg*.list
+  - '[ "$DIST" ] && sudo sed -i -e "s/pgdg.*/pgdg-testing main $PG_SUPPORTED_VERSIONS/" /etc/apt/sources.list.d/pgdg*.list'
   - sudo apt-get -qq update
 
 install: