Removed dated reference to subversion

The subversion requirement was introduced in
ba331316b0cff1d71fb477487da93be0b8494b8d for checking out pypcap.
Commit 1b53eb99e6c384b7f43f45f306e209894c8476bb switch to using a git url.
Merge pull request #255 from anadahz/fix_README

Removed dated reference to subversion