Add support for ruby file: Gemfile syntax

Previously, this would yield an error:

    appraise "foo" do
      ruby file: '../.ruby-version'

      gem "faraday-follow_redirects", "0.3.0"
    end

`Appraisal::BundlerDSL#ruby_version` is emitting a block when the
Gemfile DSL ruby method is used. Currently, this is emitted, which is
not a method argument but rather a block:

    ruby {:file=>"../.ruby-version"}

To fix this, we need to add `()` to ensure the arguments stay arguments:

    ruby({:file=>"../.ruby-version"})

This has been tested on Bundler 2.3.27 only — at this point in time we
have compatibility issues with others.

thoughtbot#218

Author: tisba

lib/appraisal/bundler_dsl.rb

@@ -108,8 +108,11 @@ def source_entry
     alias_method :source_entry_for_dup, :source_entry
 
     def ruby_version_entry
-      if @ruby_version
-        "ruby #{@ruby_version.inspect}"
+      return unless @ruby_version
+
+      case @ruby_version
+      when String then "ruby #{@ruby_version.inspect}"
+      else "ruby(#{@ruby_version.inspect})"
       end
     end
 

spec/acceptance/appraisals_file_bundler_dsl_compatibility_spec.rb

@@ -149,4 +149,33 @@
       gemspec :path => "../sitepress"
     Gemfile
   end
+
+  it 'supports ruby file: ".ruby-version" DSL' do
+    build_gemfile <<-Gemfile
+      source 'https://rubygems.org'
+
+      ruby RUBY_VERSION
+
+      gem 'appraisal', :path => #{PROJECT_ROOT.inspect}
+    Gemfile
+
+    build_appraisal_file <<-Appraisals
+      appraise 'ruby-version' do
+        ruby file: ".ruby-version"
+      end
+    Appraisals
+
+    run 'bundle install --local'
+    run 'appraisal generate'
+
+    expect(content_of 'gemfiles/ruby_version.gemfile').to eq <<-Gemfile.strip_heredoc
+      # This file was generated by Appraisal
+
+      source "https://rubygems.org"
+
+      ruby({:file=>".ruby-version"})
+
+      gem "appraisal", :path => #{PROJECT_ROOT.inspect}
+    Gemfile
+  end
 end