Add Python 3.3 as continuous integration target

Include Python 3.3 in list of Python versions TravisCI will build and
test against.

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index 68c25bd..dd7fb63 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,6 +3,7 @@
   - 2.6
   - 2.7
   - 3.2
+  - 3.3
   - pypy
 install:
   - pip install -r requirements/development.txt -r requirements/production.txt