Bump rollbar from 2.19.3 to 2.20.0

Bumps [rollbar](https://github.com/rollbar/rollbar-gem) from 2.19.3 to 2.20.0.
- [Release notes](https://github.com/rollbar/rollbar-gem/releases)
- [Changelog](https://github.com/rollbar/rollbar-gem/blob/master/CHANGELOG.md)
- [Commits](rollbar/rollbar-gem@v2.19.3...v2.20.0)

Gemfile.lock

@@ -292,7 +292,7 @@ GEM
       mime-types (>= 1.16, < 3.0)
       netrc (~> 0.7)
     rinku (2.0.6)
-    rollbar (2.19.3)
+    rollbar (2.20.0)
       multi_json
     ruby-pushbullet (0.1.4)
       json