fix bug when using with rails5

[elfassy]

#40

[bwillis]

We're still supporting Rails back to v3.2, which doesn't have this yet. I'd propose adding a check like so:

if respond_to? :prepend_before_action
  prepend_before_action :set_version
else
  prepend_before_filter :set_version
end

What do you think?

[bwillis]

Closing this out since it's now in master, thanks!