cucumber-messages incompatible with Ruby 2.7.0

[aslakhellesoy]

It appears that the google-protobuf gem doesn't have any releases that are compatible with Ruby 2.7.0. I have tried google-protobuf version 3.8.0 (which the gemspec currently points to) as well as 3.11.2 (the latest version). It gives the following error:

LoadError:
  cannot load such file -- google/protobuf_c
# /Users/aslakhellesoy/.rbenv/versions/2.7.0/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require'
# /Users/aslakhellesoy/.rbenv/versions/2.7.0/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require'
# /Users/aslakhellesoy/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/google-protobuf-3.11.2-universal-darwin/lib/google/protobuf.rb:51:in `rescue in <top (required)>'
# /Users/aslakhellesoy/.rbenv/versions/2.7.0/lib/ruby/gems/2.7.0/gems/google-protobuf-3.11.2-universal-darwin/lib/google/protobuf.rb:48:in `<top (required)>'
# /Users/aslakhellesoy/.rbenv/versions/2.7.0/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require'
# /Users/aslakhellesoy/.rbenv/versions/2.7.0/lib/ruby/2.7.0/rubygems/core_ext/kernel_require.rb:92:in `require'
# ./lib/cucumber/messages_pb.rb:4:in `<top (required)>'

Both these versions work fine on Ruby 2.6.5. There have been incompatibilities before: protocolbuffers/protobuf#5161

We have had problems with google-protobuf for JRuby (it's poorly supported) and now this. We need to switch to a pure-ruby version, as suggested in #813

That PR links to ruby-protobuf/protobuf#408 which we should provide a fix for, or find a workaround for.

[rockorequin]

FWIW, I also ran into this error using ruby 2.7 with the latest version of google-protobuf (version 3.11.2). This command says "cannot load such file google/protobuf_c":

ruby -r google/protobuf -e'puts "hello"'

Lower down in the error message it also says "cannot load such file -- google/2.7/protobuf", so I tried symlinking the 2.6 folder to 2.7 using these commands (I'm using rvm):

cd /usr/local/rvm/gems/ruby-2.7.0/gems/google-protobuf-3.11.2-x86_64-linux/lib/google
ln -s 2.6 2.7

and this fixed the problem.