Remove requirements.txt

We cannot use simple requirements as our dependencies vary across
python versions.

Signed-off-by: Zygmunt Krynicki <zkrynicki@gmail.com>
Select propert unittest2 package based on python version

Sadly unittest2 requires unittest2py3k package on python3+
This should finally fix all tests on travis

Signed-off-by: Zygmunt Krynicki <zkrynicki@gmail.com>