Relax ffi pin

Author: JonRowe

Gemfile

@@ -59,8 +59,10 @@ elsif RUBY_VERSION < '2.0'
   gem 'ffi', '< 1.9.19' # ffi dropped Ruby 1.8 support in 1.9.19
 elsif RUBY_VERSION < '2.3.0'
   gem 'ffi', '~> 1.12.0'
-else
+elsif RUBY_VERSION < '2.6.0'
   gem 'ffi', '~> 1.15.0'
+else
+  gem 'ffi', '~> 1.17.0'
 end
 
 if RUBY_VERSION < '2.3.0' && !!(RbConfig::CONFIG['host_os'] =~ /cygwin|mswin|mingw|bccwin|wince|emx/)