Remove support for Internet Explorer

"DevDocs being a developer tool, the browser requirements are high" (README)

lib/app.rb

@@ -146,10 +146,8 @@ class App < Sinatra::Application
       @browser ||= Browser.new(request.user_agent)
     end
 
-    UNSUPPORTED_IE_VERSIONS = %w(6 7 8 9).freeze
-
     def unsupported_browser?
-      browser.ie? && UNSUPPORTED_IE_VERSIONS.include?(browser.version)
+      browser.ie?
     end
 
     def docs