Bump ffi from 1.9.14 to 1.13.1

Bumps [ffi](https://github.com/ffi/ffi) from 1.9.14 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@1.9.14...1.13.1)

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

Author: dependabot[bot]

Gemfile.lock

@@ -32,7 +32,7 @@ GEM
     erubis (2.7.0)
     eventmachine (1.2.0.1)
     execjs (2.7.0)
-    ffi (1.9.14)
+    ffi (1.13.1)
     haml (5.1.2)
       temple (>= 0.8.0)
       tilt