Pin ffi to 1.12.2 due to Windows crash

See ffi/ffi#784

Signed-off-by: James Stocks <jstocks@chef.io>

Gemfile

@@ -9,7 +9,7 @@ gem "inspec", path: "."
 # in it in order to package the executable. Hence the odd backwards dependency.
 gem "inspec-bin", path: "./inspec-bin"
 
-gem "ffi", ">= 1.9.14"
+gem "ffi", "1.12.2" # 1.13 does not work on Windows: https://github.com/ffi/ffi/issues/784
 
 group :omnibus do
   gem "rb-readline"