Upgrade plugin for redmine 4.0.x with rails 5

[neffets]

Redmine 4.0.x now needs rails 5.x

some funtions and uses are now deprecated (ActionDispatch::Callbacks, or "to_prepare", ...)
aim: make the plugin ready for redmine 4.x

[neffets]

"master" branch is now running with rails5 under redmine 4.0.4.
I removed old code for redmine2 (made before this a compatibility branch "compatible-redmine2")

I will maintain for a while a separate branch "compatible-redmine3"
At this moment so only difference to master is the Gemfile: the gem "record_tag_helper" is needed since redmine4
I dont know how to declare is in the Gemfile for redmine4 only, so at now I need two branches.