Update .travis.yml

Author: computerfr33k

.travis.yml

@@ -4,6 +4,9 @@ services:
   - docker
 
 script:
+  - if [ "$TRAVIS_BRANCH" == "master" ]; then
+    export TRAVIS_BRANCH="latest";
+    fi
   - docker build --build-arg BUILD_DATE=`date -u +"%Y-%m-%dT%H:%M:%SZ"` --build-arg VCS_REF="$TRAVIS_COMMIT" -t computerfr33k/burp-server:"$TRAVIS_BRANCH" .
 
 after_success: