Add PyPI badges to README

[hugovk]

As suggested in #1634:

Pypi seems to be still at 19.7.1 https://pypi.python.org/pypi/gunicorn
As a side thought could the README.md be made to have stable version number?
going trough pypi to check what is the current release feels bit funky, or perhaps I'm missing some obvious version number displayed somewhere 😕

Preview:

Also upgrade the Travis badge to SVG which looks better, and whilst we're here, update HTTP links to HTTPS.

[berkerpeksag]

LGTM, I'll wait for a bit to see what @benoitc and @tilgovi think about this.

Thanks!

[berkerpeksag]

Thanks.