Freeze default virtualenv package install versions.

We've also converted to use a requirements file. This makes it easy to
update requirements in an out-of-date virtualenv.
Adapt to Powerline 1.0.

- The 'name' key was sort-of replaced by the 'function' key. See
  <https://github.com/Lokaltog/powerline/issues/1031> for the details.
- The 'lightyellow' color was removed, so I just changed it to
  'brightyellow'. Doesn't fit as well, but oh well. Might change it
  later.