Bump ffi from 1.9.10 to 1.13.1

Bumps [ffi](https://github.com/ffi/ffi) from 1.9.10 to 1.13.1.
- [Release notes](https://github.com/ffi/ffi/releases)
- [Changelog](https://github.com/ffi/ffi/blob/master/CHANGELOG.md)
- [Commits](ffi/ffi@1.9.10...1.13.1)

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

Author: dependabot[bot]

Gemfile.lock

@@ -2,7 +2,7 @@ GEM
   remote: https://rubygems.org/
   specs:
     colorator (0.1)
-    ffi (1.9.10)
+    ffi (1.13.1)
     jekyll (3.1.6)
       colorator (~> 0.1)
       jekyll-sass-converter (~> 1.0)