resolve build errors on travis-ci

* travis-ci no longer supports python 2.5

  http://blog.travis-ci.com/2013-11-18-upcoming-build-environment-updates/

  > Python 2.5 has been removed due to very low overall usage and
  > breaking changes in pip and virtualenv.

* GitPython

  pip install no longer supports pre-release versions by default
  http://www.pip-installer.org/en/latest/reference/pip_install.html#pre-release-versions

  resolves build error:

    $ pip install 'GitPython >= 0.2.0' --use-mirrors
    --use-mirrors has been deprecated and will be removed in the future.
    Explicit uses of --index-url and/or --extra-index-url is suggested.
    Downloading/unpacking GitPython>=0.2.0
      Could not find a version that satisfies the requirement
      GitPython>=0.2.0 (from versions: 0.1.7, 0.2.0-beta1, 0.3.0-beta1,
      0.3.0-beta2, 0.3.1-beta2, 0.3.2.RC1)
    Cleaning up...
    No distributions matching the version for GitPython>=0.2.0

.travis.yml

@@ -1,8 +1,7 @@
 language: python
 python:
-  - "2.5"
   - "2.6"
   - "2.7"
 
-install: pip install 'GitPython >= 0.2.0' --use-mirrors
+install: pip install 'GitPython >= 0.2.0' --pre
 script: nosetests test/unit