Bump ffi from 1.1.5 to 1.13.1 in /edx_specs

Bumps [ffi](https://github.com/ffi/ffi) from 1.1.5 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@1.1.5...1.13.1)

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

Author: dependabot[bot]

edx_specs/Gemfile.lock

@@ -21,7 +21,7 @@ GEM
       i18n (~> 0.4)
     faye-websocket (0.4.6)
       eventmachine (>= 0.12.0)
-    ffi (1.1.5)
+    ffi (1.13.1)
     http_parser.rb (0.5.3)
     i18n (0.6.0)
     json (1.7.4)