rubocop: disable InsecureHashAlgorithm check

We use sha1 as a checksum for deciding when pages are out of date (so a
weak algorithm is fine). This is really independent of Git's use of
sha1, so I think it could just be changed. But it would probably involve
a complete re-import of all of the pages. So let's just ignore it for
the purposes of this PR.

Author: peff

.rubocop.yml

@@ -21,3 +21,6 @@ Rails/HelperInstanceVariable:
 
 Rails/TimeZone:
   Enabled: false
+
+GitHub/InsecureHashAlgorithm:
+  Enabled: false