Update .travis.yml

Doh, the shebang line for run-tests itself relies on `python3`

.travis.yml

@@ -5,7 +5,7 @@ compiler:
 before_script:
   - sudo add-apt-repository -y ppa:fkrull/deadsnakes
   - sudo apt-get update -qq
-  - sudo apt-get install -y python3.3
+  - sudo apt-get install -y python3.3 python3
 
 script:
   - make -C unix