requirements.txt: Typo fix

comparation: 2. (nonstandard) comparison
 -- http://en.wiktionary.org/wiki/comparation
Add pytz to requirements.txt

An explicit version of pytz is needed, because new versions of
pip refuse to install development versions of packages, and the
pytz versioning scheme looks like it's all development versions.

For details see pip bug pypa/pip#974:
https://github.com/pypa/pip/issues/974
and pytz bug #1204837:
https://bugs.launchpad.net/pytz/+bug/1204837
Merge pull request #71 from encukou/requirements-fixes

requirements.txt