travis: Add dummy configuration for master

Apparently we need a dummy travis configuration on the master branch, so that we can tell it not to build the master branch.

.travis.yml

@@ -0,0 +1,7 @@
+language: cpp
+branches:
+  only:
+    - release/1.10.x
+    - release/1.9.x
+script:
+    - echo "Build disabled on master branch."