Make the `pip install` a single line/command.
Spell out "-U" and "-I" in the `pip install` instruction.
Use "--use-mirrors" for the `pip install` instruction.

"Use the PyPI mirrors as a fallback in case the main index is down."

Also recommended by the Travis project:
"We highly recommend using `--use-mirrors` if you override dependency
installation command to reduce the load on PyPI and possibility of
installation failures."
( http://about.travis-ci.org/docs/user/languages/python/#Travis-CI-uses-pip )
Update Travis config to reflect changes in README.
Merge pull request #31 from Freso/change-pip-instructions

Update `pip install` instructions.

1. Only use a single `pip install` command to install dependencies.
2. Spell out the `-U` and `-I` parameters for more readability/transparency.
3. Use PyPI mirrors (`--use-mirrors`).