Downgrade bcrypt to v3.1.12

v3.1.13 doesn't work compile on ARM platforms:
https://github.com/codahale/bcrypt-ruby/issues/201

We can revert the change in
https://gitlab.com/gitlab-org/gitlab/merge_requests/18225, since this
wasn't an essential update.

See https://gitlab.com/gitlab-org/omnibus-gitlab/issues/4802

Gemfile.lock

@@ -95,7 +95,7 @@ GEM
     babosa (1.0.2)
     base32 (0.3.2)
     batch-loader (1.4.0)
-    bcrypt (3.1.13)
+    bcrypt (3.1.12)
     bcrypt_pbkdf (1.0.0)
     benchmark-ips (2.3.0)
     benchmark-memory (0.1.2)