Remove Python 2.5 from .travis.yml.

Travis CI no longer supports Python 2.5.  Thanks to @dmorrison42
for pointing this out.  Removing 2.5 from the Travis config should
make the Travis build status image work again.

.travis.yml

@@ -1,8 +1,7 @@
 language: python
 
-# Travis CI has no plans to support Python 2.4 and Jython.
+# Travis CI has no plans to support Jython and no longer supports Python 2.5.
 python:
-  - 2.5
   - 2.6
   - 2.7
   - 3.2