Cache GitHub responses in memcached

https://trello.com/c/1qeitvaD/204-address-rate-limiting
Revert "Cache GitHub responses in memcached"

* Seeing an encoding exception on staging

  > Encoding::UndefinedConversionError: "\xE2" from ASCII-8BIT to UTF-8

This reverts commit 57f6f6372f418e9944440d7d98ad96c5d35dd151.