Force FFI version 1.12 to compile on OSX

See ffi/ffi#791

Author: ChrisBAshton

Gemfile

@@ -10,3 +10,7 @@ gem 'tzinfo-data', platforms: [:mswin, :mingw, :jruby]
 
 # Include the tech docs gem
 gem 'govuk_tech_docs'
+
+# Include specific version of FFI until following issue is fixed:
+# https://github.com/ffi/ffi/issues/791
+gem "ffi", "1.12"

Gemfile.lock

@@ -43,7 +43,7 @@ GEM
     execjs (2.7.0)
     fast_blank (1.0.0)
     fastimage (2.2.0)
-    ffi (1.13.1)
+    ffi (1.12.0)
     govuk_tech_docs (2.0.13)
       activesupport
       chronic (~> 0.10.2)
@@ -174,6 +174,7 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
+  ffi (= 1.12)
   govuk_tech_docs
   tzinfo-data
   wdm (~> 0.1.0)