Balancer is broken in 1.5.8

[ambentzen]

The proxy/balancer is broken in 1.5.8 as it uses the transform_keys method on Hash, which was as far as I can tell was introduced in Ruby 2.5.5.

It is used here: https://github.com/code-mancers/invoker/blob/1992f355ad82470d172f7fee6078cbb4817f61cb/lib/invoker/power/balancer.rb#L68

If I manually specify the port in the URL I can access my app.

Version 1.5.7 seems to work, for me at least, on Ruby 2.3.8.