update submodule urls to public urls

Some submodule urls were of the form `git@github.com:owner/example.git`
which requires a public key to download. Converting these to the public url format
`https://github.com/owner/example.git` allows them to update without requiring the public key

Change based on http://stackoverflow.com/questions/8197089/fatal-error-when-updating-submodule-using-git#answer-14807969

Addresses Issue #2