made separate funcs for official EMR host and the one that works with SSL
made RetryGoRound wrapper and tested it
added fallback host for EMR
updated endpoint tests to use "official" endpoint again
added test for SSL fallback
use more readable format strings for AWS endpoints
make RetryGoRound.__alternatives a tuple
Merge pull request #716 from davidmarin/ssl_go_round

Only fall back to EMR SSL host if we need to (fixes #706).