Rails6 compatibility

[fwolfst]

In Rails6 the signature for TemplateHandlers apparently changed.

#call now requires two arguments (template and source). I will PR an easy fix, but do not know how to keep backwards compatibility (other template handler implementations should have done that, though).

[fwolfst]

Problem seemed to be a redcarpet dependency. I had redcarpet 3.5 which apparently is only compatible with emd 0.1 .

I propose:

• check and relax the redcarpet dependency in gemspec ( '~> 3') or
• state the versions explicitely in README:

    gem 'emd', '~> 0.5'
    gem 'redcarpet', '~> 3.4'

[fwolfst]

Again summarized: The fixes in 0.5.0 (#13) work for me, but bundle and bundle update did not fetch it.