Fix dyld: lazy symbol binding failed: Symbol not found error

Build fails with:

```
dyld: lazy symbol binding failed: Symbol not found: _ffi_prep_closure_loc
  Referenced from: /Users/christopherashton/.rbenv/versions/2.6.6/lib/ruby/gems/2.6.0/gems/ffi-1.13.1/lib/ffi_c.bundle
  Expected in: /usr/lib/libffi.dylib

dyld: Symbol not found: _ffi_prep_closure_loc
  Referenced from: /Users/christopherashton/.rbenv/versions/2.6.6/lib/ruby/gems/2.6.0/gems/ffi-1.13.1/lib/ffi_c.bundle
  Expected in: /usr/lib/libffi.dylib

[1]    66481 abort      GITHUB_TOKEN=$(cat ~/github_token.txt) NO_CONTRACTS=true bundle exec middlema
```

Downgrading to lower version fixes it.
It looks as though this should have been fixed in
ffi/ffi#787, but we're seeing the same
error as originally reported.

Gemfile.lock

@@ -63,7 +63,7 @@ GEM
       faraday (~> 1.0)
     fast_blank (1.0.0)
     fastimage (2.1.7)
-    ffi (1.13.1)
+    ffi (1.12.2)
     github-markdown (0.6.9)
     govuk_schemas (4.0.0)
       json-schema (~> 2.8.0)