Adjusted travis config for new branching

.travis.yml

 matrix:
   include:
     - name: "Master build"
-      if: branch = master
+      if: branch = flutter-master
       env: FLUTTER_VERSION=master
     - name: "Stable build"
-      if: branch != master
+      if: branch != flutter-master
       env: FLUTTER_VERSION=stable
 
 before_script: