Use Miniconda to install dependencies on Travis CI

Currently, Travis CI builds are failing during the build of
scipy. Output:

    Command /home/travis/virtualenv/python2.6.9/bin/python -c "import setuptools, tokenize;__file__='/home/travis/virtualenv/python2.6.9/build/scipy/setup.py';exec(compile(getattr(tokenize, 'open', open)(__file__).read().replace('\r\n', '\n'), __file__, 'exec'))" install --record /tmp/pip-kSzM2D-record/install-record.txt --single-version-externally-managed --compile --install-headers /home/travis/virtualenv/python2.6.9/include/site/python2.6 failed with error code 1 in /home/travis/virtualenv/python2.6.9/build/scipy
    Traceback (most recent call last):
      File "/home/travis/virtualenv/python2.6.9/bin/pip", line 11, in <module>
        sys.exit(main())
      File "/home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages/pip/__init__.py", line 185, in main
        return command.main(cmd_args)
      File "/home/travis/virtualenv/python2.6.9/lib/python2.6/site-packages/pip/basecommand.py", line 161, in main
        text = '\n'.join(complete_log)
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xe2 in position 40: ordinal not in range(128)
    The command "pip install ." failed and exited with 1 during .

To circumvent this, we're going to use a popular approach to installing
python dependencies on Travis CI, as demonstrated in this GitHub Gist:

    https://gist.github.com/dan-blanchard/7045057