Bump ffi from 1.9.18 to 1.15.0 (heroku#74)

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>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -57,7 +57,7 @@ GEM
     crass (1.0.5)
     erubi (1.8.0)
     execjs (2.7.0)
-    ffi (1.9.18)
+    ffi (1.15.0)
     globalid (0.4.2)
       activesupport (>= 4.2.0)
     i18n (1.6.0)