[grilo] travis: do not push container without credentials

[Juan A. Suárez Romero <jasuarez src gnome org>]

commit 9511ba62ea0c944cd596704d31bdc516091e1610
Author: Juan A. Suarez Romero <jasuarez igalia com>
Date:   Sun Feb 12 03:26:22 2017 +0100

    travis: do not push container without credentials
    
    Avoid trying to push container to Docker Hub if no credentials have been
    defined.

 .travis.yml |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/.travis.yml b/.travis.yml
index 7c903ff..37357a6 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -27,7 +27,7 @@ script:
   - docker build -t grilofw/grilo:core .
   
 after_success:
-  - if [[ "$TRAVIS_BRANCH" == "master" ]]; then
+  - if [[ "$TRAVIS_BRANCH" == "master" && -n "$DOCKER_USERNAME" ]]; then
       docker login -u "$DOCKER_USERNAME" -p "$DOCKER_PASSWORD";
       docker push grilofw/grilo:core;
     fi