Fixed fallback curl github API call

The fallback 'curl' method used if ST doesn't include SSL support fails
because of unneeded escape and quotes: escaping the json message text
payload with \" and \` and surrounding it with '"' gives a
'Problems parsing JSON' error from github API.

Special character substitution and double quotes were removed from the
markdown source text sent via subprocess.Popen()

Moreover, the github_oauth_token is used for authentication, when
configured.