Restrict travis build to master branch.

We want the build badge on the front page to be always green. Branches can fail, that's fine, but currently this gives the impression that all of assimp is failing. Branches should do pull requests against master to get a CI build. Such PRs can be marked "do not merge" and updated indefinitely.

.travis.yml

@@ -1,6 +1,10 @@
 before_install:
   - sudo apt-get install cmake python3
 
+branches:
+  only:
+    - master
+
 env:
     matrix:
     - LINUX=1 TRAVIS_NO_EXPORT=YES