Handle both D7 and D8
Update README for D7

registry_rebuild is not needed for D8. Also adding backticks around
shell commands and showing cd'ing to drush directory because I've seen
folks give up after trying to download registry_rebuild from their
Drupal install.
Merge pull request #3 from Cottser/d7-compatibility

- Pull #3
  * Add Drupal 7 compatibility