Relax ffi dependency

Author: dalibor

cobhan.gemspec

@@ -35,5 +35,5 @@ Gem::Specification.new do |spec|
   spec.executables = spec.files.grep(%r{\Aexe/}) { |f| File.basename(f) }
   spec.require_paths = ['lib']
 
-  spec.add_dependency 'ffi', '~> 1.15.4'
+  spec.add_dependency 'ffi'
 end