Bump ffi from 1.9.14 to 1.15.4 in /bootstrap

Bumps [ffi](https://github.com/ffi/ffi) from 1.9.14 to 1.15.4.
- [Release notes](https://github.com/ffi/ffi/releases)
- [Changelog](https://github.com/ffi/ffi/blob/master/CHANGELOG.md)
- [Commits](https://github.com/ffi/ffi/compare/1.9.14...v1.15.4)

---
updated-dependencies:
- dependency-name: ffi
  dependency-type: indirect
...

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

bootstrap/Gemfile.lock

@@ -3,7 +3,8 @@ GEM
   specs:
     addressable (2.4.0)
     colorator (0.1)
-    ffi (1.9.14-x64-mingw32)
+    ffi (1.15.4)
+    ffi (1.15.4-x64-mingw32)
     jekyll (3.1.6)
       colorator (~> 0.1)
       jekyll-sass-converter (~> 1.0)
@@ -33,6 +34,7 @@ GEM
     sass (3.4.22)
 
 PLATFORMS
+  ruby
   x64-mingw32
 
 DEPENDENCIES