Download links in README

[wolfiex]

These should be corrected to the https links to avoid permissions errors on some machines.

 git clone [email protected]:snap-stanford/snap-python.git
 git clone [email protected]:snap-stanford/snap.git

git clone https://github.com/snap-stanford/snap-python.git
git clone https://github.com/snap-stanford/snap.git

[wolfiex]

Also it might be worth mentioning that it is possible to install swig using conda, which should work for most systems.