Bump oauth from 0.5.4 to 0.5.6

Bumps [oauth](https://github.com/oauth-xx/oauth-ruby) from 0.5.4 to 0.5.6.
- [Release notes](https://github.com/oauth-xx/oauth-ruby/releases)
- [Changelog](https://github.com/oauth-xx/oauth-ruby/blob/master/HISTORY)
- [Commits](https://github.com/oauth-xx/oauth-ruby/compare/v0.5.4...v0.5.6)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -10,7 +10,7 @@ GEM
     multi_json (1.15.0)
     mustermann (1.1.1)
       ruby2_keywords (~> 0.0.1)
-    oauth (0.5.4)
+    oauth (0.5.6)
     pry (0.14.1)
       coderay (~> 1.1)
       method_source (~> 1.0)