Bump sinatra from 2.0.7 to 2.0.8.1

Bumps [sinatra](https://github.com/sinatra/sinatra) from 2.0.7 to 2.0.8.1.
- [Release notes](https://github.com/sinatra/sinatra/releases)
- [Changelog](https://github.com/sinatra/sinatra/blob/master/CHANGELOG.md)
- [Commits](sinatra/sinatra@v2.0.7...v2.0.8.1)

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

Author: dependabot-preview[bot]

Gemfile.lock

@@ -1,16 +1,18 @@
 GEM
   remote: http://rubygems.org/
   specs:
-    mustermann (1.0.3)
+    mustermann (1.1.0)
+      ruby2_keywords (~> 0.0.1)
     rack (2.0.8)
-    rack-protection (2.0.7)
+    rack-protection (2.0.8.1)
       rack
     redis (4.1.3)
+    ruby2_keywords (0.0.1)
     simple-random (1.0.3)
-    sinatra (2.0.7)
+    sinatra (2.0.8.1)
       mustermann (~> 1.0)
       rack (~> 2.0)
-      rack-protection (= 2.0.7)
+      rack-protection (= 2.0.8.1)
       tilt (~> 2.0)
     split (3.4.1)
       redis (>= 2.1)