Update ffi dependency

Author: Paxa

fast_excel.gemspec

@@ -15,6 +15,6 @@ Gem::Specification.new do |s|
   s.require_paths = ["lib"]
   s.extensions = ["extconf.rb"]
 
-  s.add_runtime_dependency "ffi", ["> 1.9", "< 2"]
+  s.add_runtime_dependency "ffi", ["> 1.17", "< 2"]
   s.add_runtime_dependency "base64", [">= 0.2", "< 2"]
 end