Limit pygit2 gitfs provider transports

pygit2 doesn't yet support ssh transports properly, as seen here:

http://stackoverflow.com/questions/20956895/how-do-i-set-ssh-credentials-for-libgit2-libssh2

This commit limits pygit2 to http://, https://, and file:// URIs. Once a
compatible pygit2 version is released, we can do version checking and
allow ssh-based transports for pygit2 on supported versions.