Fix Travis for Rails 5.2.2.1
Fix Travis for Rails 5.2.2.1

Special setup for combustion for Rails 5.2 seems to be no longer needed in 5.2.2.1 -- and worse, causes a problem in Rails 5.2.2.1. Tests seem to pass fine without it though.

See https://github.com/pat/combustion/issues/97

jrochkind/json_attribute,jrochkind/json_attribute,jrochkind/json_attribute,jrochkind/json_attribute