Add cache clear command to Quick Demo.

It might be useful to remind people of clearing the drush cache if the pantheon commands don't appear.
Switch terminus_session_data() to terminus_bootstrap() in drush_terminus_pantheon_site_make_backup() function so that terminus.backup.api.inc is included before calling terminus_api_site_make_backup().
Merge branch 'master' of github.com:benbunk/terminus
Merge pull request #6 from benbunk/master

Add cache clear command to Quick Demo section in README.md