Upgrade requests library to version 2.1.1

Upgrade the `requests` library to match the version provided by Ubuntu
Trusty, version 2.1.1.

This is to prevent a conflict on Ubuntu Trusty between system Python
libraries and Pip libraries.

Specifically, Pip relies on the `IncompleteRead` module that is exported
by `requests.compat`. Version 2.4.3 of the `requests` library removed
that exported module[1].

When `ghtools` is installed, Pip would upgrade `requests` to version
2.4.3 (the latest available), thereby causing Pip to break because the
`requests` module installed by Pip (in
`/usr/lib/python2.7/dist-packages/`) takes precendence over the system
version of that module.

This was causing the following Puppet error on our ci-slave-4 box in Vagrant
using Ubuntu Trusty:

    ==> ci-slave-4: Error: Could not prefetch package provider 'pip': [nil, nil, [(provider=pip)], nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil, nil]

When I tried to run `pip` on the Vagrant box, I got the following error:

    vagrant@ci-slave-4:~$ pip
    Traceback (most recent call last):
      File "/usr/bin/pip", line 9, in <module>
        load_entry_point('pip==1.5.4', 'console_scripts', 'pip')()
      File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 351, in load_entry_point
        return get_distribution(dist).load_entry_point(group, name)
      File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 2363, in load_entry_point
        return ep.load()
      File "/usr/lib/python2.7/dist-packages/pkg_resources.py", line 2088, in load
        entry = __import__(self.module_name, globals(),globals(), ['__name__'])
      File "/usr/lib/python2.7/dist-packages/pip/__init__.py", line 11, in <module>
        from pip.vcs import git, mercurial, subversion, bazaar  # noqa
      File "/usr/lib/python2.7/dist-packages/pip/vcs/mercurial.py", line 9, in <module>
        from pip.download import path_to_url
      File "/usr/lib/python2.7/dist-packages/pip/download.py", line 25, in <module>
        from requests.compat import IncompleteRead
    ImportError: cannot import name IncompleteRead
    vagrant@ci-slave-4:~$

By installing requiring the exact same version of `requests` as the one
provided by the system under Ubuntu Trusty, Pip no longer needs to
install the `requests` module and `ghtools` will use the system module.
This is also tested under Precise and does not break Pip.

I verified the version of `requests` installed on Ubuntu Trusty prior to
installing `ghtools`:

    vagrant@jumpbox-2:~/ghtools$ python
    Python 2.7.6 (default, Mar 22 2014, 22:59:56)
    [GCC 4.8.2] on linux2
    Type "help", "copyright", "credits" or "license" for more information.
    >>> import requests
    >>> print requests.__version__
    2.2.1
    >>>

When installing `ghtools`, Pip detects the existing system version and
will not install its own:

    vagrant@jumpbox-2:~/ghtools$ sudo pip install .
    Unpacking /home/vagrant/ghtools
      Running setup.py (path:/tmp/user/0/pip-0GLR1h-build/setup.py) egg_info for package from file:///home/vagrant/ghtools

    Requirement already satisfied (use --upgrade to upgrade): requests==2.2.1 in /usr/lib/python2.7/dist-packages (from ghtools==0.21.0)
    [...]

For more context, please see this bug report (though the bug is not in
python-pip):
https://bugs.launchpad.net/ubuntu/+source/python-pip/+bug/1306991

[1]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=766419
Merge pull request #9 from alphagov/upgrade_requests_library_2

[#82935274] Upgrade requests library to version 2.1.1