Use more succint #then alias instead of #yield_self

`Object#yield_self` is available from Ruby 2.6 [1]

[1] - https://ruby-doc.org/core-2.6/Object.html#method-i-yield_self

core/lib/generators/solidus/install/app_templates/frontend/break_down_solidus_gem.rb

@@ -18,7 +18,7 @@ def build_gem_lines(conservative_versioning)
       requirement = if is_local
                       ", path: \"#{d.source.path}\""
                     elsif is_git
-                      ", git: \"#{d.git}\"".yield_self { |g| d.ref ? g + ", ref: \"#{d.ref}\"" : g }
+                      ", git: \"#{d.git}\"".then { |g| d.ref ? g + ", ref: \"#{d.ref}\"" : g }
                     elsif conservative_versioning
                       ", \"#{conservative_version(@definition.specs[d.name][0])}\""
                     else