windows wheel test fix
switch to an unbroken bitbucket project for tests
explicitly require virtualenv>=1.9.1 for travis
remove unnecessary markerlib installs in wheel tests
Fix issue #32: pip crashes when server does not send content-type header

Thanks Kelsey Hightower for pointing what to change.
Change test to use set() comparison and add docstring

Using set because and os.listdir() does not guarantee ordering.
Merge pull request #872 from hltbra/issue-32

Fix issue #32: pip crashes when server does not send content-type header
update changelog