Simplify Travis build

This should make the build faster since we don't have to wait until we
get our hands on five build machines. The log will be simpler on both
travis-ci.org and coveralls.io. If it becomes too simple, we'll revert
this later.

.travis.yml

@@ -1,10 +1,6 @@
 language: python
 env:
-  - ENV=py27 COVERAGE=on
-  - ENV=py33 COVERAGE=on
-  - ENV=py34 COVERAGE=on
-  - ENV=pep8
-  - ENV=pyflakes
+  - COVERAGE=on
 install: pip install tox python-coveralls
-script: tox -e $ENV
-after_success: test -n "$COVERAGE" && coveralls
+script: tox
+after_success: coveralls