Relax highline dependency

brakeman.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |s|
   s.add_dependency "ruby2ruby", "~>2.1.1"
   s.add_dependency "terminal-table", "~>1.4.5"
   s.add_dependency "fastercsv", "~>1.5"
-  s.add_dependency "highline", "~>1.6.20"
+  s.add_dependency "highline", "~>1.6"
   s.add_dependency "erubis", "~>2.6"
   s.add_dependency "haml", ">=3.0", "<5.0"
   s.add_dependency "sass", "~>3.0"