Add environment as an argument for pantheon-site-backups.

The callback for pantheon-site-backups could take environment but it doesn't require it in the drush command and it wasn't checked for in the function which creates this error when called without the environment: 

Bens-MacBook-Pro:terminus ben$ drush pantheon-site-backups $SITE-ID
Request failed: 404 Not found.                                                                               [failed]
Invalid argument supplied for foreach() terminus.drush.inc:404                                               [warning]
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