Bump ffi from 1.9.18 to 1.13.1

Bumps [ffi](https://github.com/ffi/ffi) from 1.9.18 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@1.9.18...1.13.1)

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

Author: dependabot[bot]

Gemfile.lock

@@ -117,7 +117,7 @@ GEM
     execjs (2.7.0)
     faraday (0.12.1)
       multipart-post (>= 1.2, < 3)
-    ffi (1.9.18)
+    ffi (1.13.1)
     globalid (0.4.2)
       activesupport (>= 4.2.0)
     i18n (0.9.5)