Allow spidering

I only put this in two days ago 🤦

I need this to be allowed in order to review structured data in checkers such
as https://search.google.com/structured-data/testing-tool/u/0/ and
https://search.google.com/test/rich-results

This only affects review apps as this robots.txt is not publicly accessible
from production-like environments

We noindex, nofollow all the content pages in Heroku review apps, so I think
we're safe to allow robots to access these pages.

public/robots.txt

@@ -1,5 +1,5 @@
 # See http://www.robotstxt.org/robotstxt.html for documentation on how to use the robots.txt file
 #
 # To ban all spiders from the entire site uncomment the next two lines:
-User-agent: *
-Disallow: /
+# User-agent: *
+# Disallow: /