Use ruby2_keywords for the moment (activeadmin#205)

Without this gem, current activeadmin version becomes incompatible with
the master branch of `arbre`. It shouldn't be an issue as long as we
update activeadmin's code to use keyword arguments everywhere, set a
minimum dependency on the next `arbre` and release both gems at the same
time, but I think it's easier to be conservative.

Author: deivid-rodriguez

arbre.gemspec

@@ -18,5 +18,8 @@ Gem::Specification.new do |s|
   s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
   s.require_paths = ["lib"]
 
+  s.required_ruby_version = '>= 2.5'
+
   s.add_dependency("activesupport", ">= 3.0.0")
+  s.add_dependency("ruby2_keywords", ">= 0.0.2")
 end

lib/arbre/context.rb

@@ -1,4 +1,5 @@
 require 'arbre/element'
+require 'ruby2_keywords'
 
 module Arbre
 
@@ -74,7 +75,7 @@ def respond_to_missing?(method, include_all)
     # Webservers treat Arbre::Context as a string. We override
     # method_missing to delegate to the string representation
     # of the html.
-    def method_missing(method, *args, &block)
+    ruby2_keywords def method_missing(method, *args, &block)
       if cached_html.respond_to? method
         cached_html.send method, *args, &block
       else

lib/arbre/element.rb

@@ -1,6 +1,7 @@
 require 'arbre/element/builder_methods'
 require 'arbre/element/proxy'
 require 'arbre/element_collection'
+require 'ruby2_keywords'
 
 module Arbre
 
@@ -191,7 +192,7 @@ def clear_children!
     #  3. Call the method on the helper object
     #  4. Call super
     #
-    def method_missing(name, *args, &block)
+    ruby2_keywords def method_missing(name, *args, &block)
       if current_arbre_element.respond_to?(name)
         current_arbre_element.send name, *args, &block
       elsif assigns && assigns.has_key?(name)
@@ -207,7 +208,7 @@ def method_missing(name, *args, &block)
     # which will be rendered (#to_s) inside ActionView::Base#capture.
     # We do not want such elements added to self, so we push a dummy
     # current_arbre_element.
-    def helper_capture(name, *args, &block)
+    ruby2_keywords def helper_capture(name, *args, &block)
       s = ""
       within(Element.new) { s = helpers.send(name, *args, &block) }
       s.is_a?(Element) ? s.to_s : s