Travis: explicitly install python3.4-dev

Author: Clement Champetier

.travis.yml

@@ -84,7 +84,7 @@ addons:
     - python-imaging
     - python-numpy
     - python-nose
-    - python3-dev
+    - python3.4-dev
     - python3-setuptools
     - python3-nose
     - libfreetype6-dev