Bump multipart-post from 2.0.0 to 2.1.1

Bumps [multipart-post](https://github.com/nicksieger/multipart-post) from 2.0.0 to 2.1.1.
- [Release notes](https://github.com/nicksieger/multipart-post/releases)
- [Changelog](https://github.com/socketry/multipart-post/blob/master/History.txt)
- [Commits](socketry/multipart-post@v2.0.0...v2.1.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot-support

Gemfile.lock

@@ -214,7 +214,7 @@ GEM
       jekyll-feed (~> 0.9)
       jekyll-seo-tag (~> 2.1)
     minitest (5.11.3)
-    multipart-post (2.0.0)
+    multipart-post (2.1.1)
     nokogiri (1.10.3)
       mini_portile2 (~> 2.4.0)
     octokit (4.14.0)