Changed python3 interpreter to default.

Currently, as a python3 interpreter, specific version has been chosen.
As fs-uae-wrapper doesn't use any special features from latest version
of the python3, it doesn't make sense to have fixed version of it. Just
let operating system pick right python 3 interpreter.

.travis.yml

@@ -2,7 +2,10 @@ language: python
 env:
 - TOXENV=py27
 - TOXENV=py27-flake8
-- TOXENV=py34
-- TOXENV=py34-flake8
+- TOXENV=py3
+- TOXENV=py3-flake8
 install: pip install tox
 script: tox
+before_install:
+  - sudo apt-get update
+  - sudo apt-get install -y python-tk python3-tk