Deal with new ffi Ruby version requirement (#1115)

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

Related:
- ffi/ffi#699

Gemfile

@@ -34,6 +34,8 @@ elsif RUBY_VERSION < '2'
   gem 'ffi', '< 1.10' # ffi dropped Ruby 1.8 support in 1.9.19
 elsif RUBY_VERSION < '1.9'
   gem 'ffi', '< 1.9.19' # ffi dropped Ruby 1.8 support in 1.9.19
+elsif RUBY_VERSION < '2.0'
+  gem 'ffi', '< 1.11.0' # ffi dropped Ruby 1.9 support in 1.11.0
 else
   gem 'ffi', '> 1.9.24' # prevent Github security vulnerability warning
 end