[grilo-plugins] travis: do not push container without credentials

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

commit 9e0f5e319731e6d7b4fd8713420d0119d6a2eadf
Author: Juan A. Suarez Romero <jasuarez igalia com>
Date:   Sun Feb 12 03:30:39 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, 2 insertions(+), 0 deletions(-)
---
diff --git a/.travis.yml b/.travis.yml
index 534833f..7890071 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -81,6 +81,8 @@ script:
 after_success:
   - if [[ "$BUILD" == "autotools" && "$TRAVIS_BRANCH" == "master" ]]; then
       docker run -e COVERALLS_REPO_TOKEN=$COVERALLS_REPO_TOKEN grilofw/grilo:full /bin/sh -c "cd 
/root/grilo-plugins && coveralls --exclude tests --exclude help --exclude src/lua-factory/lua-library";
+    fi
+  - if [[ "$BUILD" == "autotools" && "$TRAVIS_BRANCH" == "master" && -n "$DOCKER_USERNAME" ]]; then
       docker login -u "$DOCKER_USERNAME" -p "$DOCKER_PASSWORD";
       docker push grilofw/grilo:full;
     fi