Make Github.connect() fail if no credentials are setup

With the previous change if no credentials were defined Github.connect() would
fall back on an anonymous connection.

This commit changes the behaviour back to what it was before so that if there
are no credentials defined in the ~/.github file and no credentials defined
in the environment an IOException is thrown to alert the method caller.

The caller can call Github.connectAnonymously() if that scenario is allowed.

This should handle most cases unless callers are depending on the
FileNotFoundException being specifically thrown instead of an IOException.