Update rack-oauth2 from 1.2.1 to 1.2.3.

Removes methods deprecated in Rails 5.

https://github.com/nov/rack-oauth2/compare/v1.2.1...v1.2.3

Gemfile.lock

@@ -505,7 +505,7 @@ GEM
     rack-cors (0.4.0)
     rack-mount (0.8.3)
       rack (>= 1.0.0)
-    rack-oauth2 (1.2.1)
+    rack-oauth2 (1.2.3)
       activesupport (>= 2.3)
       attr_required (>= 0.0.5)
       httpclient (>= 2.4)