Bump ffi from 1.9.18 to 1.11.1 in /fixture_rails_root

Bumps [ffi](https://github.com/ffi/ffi) from 1.9.18 to 1.11.1.
- [Release notes](https://github.com/ffi/ffi/releases)
- [Changelog](https://github.com/ffi/ffi/blob/master/CHANGELOG.md)
- [Commits](ffi/ffi@1.9.18...1.11.1)

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

fixture_rails_root/Gemfile.lock

@@ -61,7 +61,7 @@ GEM
     concurrent-ruby (1.0.5)
     erubi (1.6.1)
     execjs (2.7.0)
-    ffi (1.9.18)
+    ffi (1.11.1)
     globalid (0.4.0)
       activesupport (>= 4.2.0)
     i18n (0.8.6)