Deal with new ffi Ruby version requirement

CI was failing for 1.9.2 and 1.9.3. This is due to
ffi/ffi#683

Related:
- ffi/ffi#699

Gemfile

@@ -44,6 +44,8 @@ if RUBY_VERSION.to_f < 2
   gem 'json', '< 2'
   gem 'term-ansicolor',  '< 1.4.0' # used by cucumber
   gem 'tins', '~> 1.6.0' # used by term-ansicolor
+  # ffi dropped Ruby 1.8 support in 1.9.19 and Ruby 1.9 support in 1.11.0
+  gem 'ffi', '< 1.9.19'
 else
   gem "cucumber"
 end