Update .travis.yml

Author: tarelli

.travis.yml

@@ -1,7 +1,7 @@
 language: java
 
 install:
-    - git clone https://github.com/openworm/org.geppetto.model.git -b development
+    - git clone https://github.com/openworm/org.geppetto.model.git -b $TRAVIS_BRANCH
     - cd org.geppetto.model
     - mvn install
     - cd ..