use starts_with? instead of start_with, fixes #253 and #251

lib/locomotive/liquid/filters/html.rb

@@ -163,7 +163,7 @@ module Locomotive
         end
 
         def absolute_url(url)
-          url.starts_with('/') ? url : "/#{url}"
+          url.starts_with?('/') ? url : "/#{url}"
         end
 
       end

lib/locomotive/middlewares/seo_trailing_slash.rb

@@ -9,7 +9,7 @@ module Locomotive
       def call(env)
         path = env['PATH_INFO']
 
-        if !path.starts_with('/admin/') && (match = path.match(%r{(.+)/$}))
+        if !path.starts_with?('/admin/') && (match = path.match(%r{(.+)/$}))
           response = Rack::Response.new
           response.redirect(match[1], 301) # moved permanently
           response.finish