Bump ffi from 1.9.18 to 1.15.0

Bumps [ffi](https://github.com/ffi/ffi) from 1.9.18 to 1.15.0.
- [Release notes](https://github.com/ffi/ffi/releases)
- [Changelog](https://github.com/ffi/ffi/blob/master/CHANGELOG.md)
- [Commits](ffi/ffi@1.9.18...v1.15.0)

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

Author: dependabot[bot]

Gemfile.lock

@@ -4,7 +4,7 @@ GEM
     addressable (2.5.1)
       public_suffix (~> 2.0, >= 2.0.2)
     colorator (1.1.0)
-    ffi (1.9.18)
+    ffi (1.15.0)
     forwardable-extended (2.6.0)
     jekyll (3.5.1)
       addressable (~> 2.4)