Replace robots.txt with robotstxt module.

[stooit]

Addresses #60.

1. Add robotstxt to drupal-org.make file
2. Add as dependency to govcms install profile
3. Remove robots.txt from docroot in phing build file after drush make has completed running
4. Add behat test to assert that robots.txt is still accessible over HTTP
5. Add behat test to assert that robots.txt is returned from robotstxt module and NOT robots.txt file on disk (checks for sitemap.xml only returned via robotstxt module)
6. Add update hook for existing govCMS sites to enable robotstxt module

[fiasco]

thanks @SRowlands!