Bump rack-timeout from 0.4.2 to 0.5.1

Bumps [rack-timeout](https://github.com/heroku/rack-timeout) from 0.4.2 to 0.5.1.
- [Release notes](https://github.com/heroku/rack-timeout/releases)
- [Changelog](https://github.com/sharpstone/rack-timeout/blob/master/CHANGELOG.md)
- [Commits](zombocom/rack-timeout@v0.4.2...v0.5.1)

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

Author: dependabot-preview[bot]

Gemfile.lock

@@ -165,7 +165,7 @@ GEM
       rack (>= 1.0.0, < 3)
     rack-test (0.6.3)
       rack (>= 1.0)
-    rack-timeout (0.4.2)
+    rack-timeout (0.5.1)
     rails (4.2.11.1)
       actionmailer (= 4.2.11.1)
       actionpack (= 4.2.11.1)