Switch from python-coveralls to coveralls for 2.6 support

.travis.yml

@@ -7,7 +7,7 @@ python:
   - "3.3"
 install:
   - "pip install nose"
-  - "pip install python-coveralls"
+  - "pip install coveralls"
   - "curl -s http://www.defunct.cc/maxmind-geoip-samples.tar.gz | tar -xzC tests/"
 script:
   - nosetests --with-coverage --cover-package=pygeoip