Allow "ruby file:" syntax in Gemfile

This allows the use of the `ruby file:` syntax in
Gemfiles, e.g.:

```ruby
ruby file: "../.ruby-version"

…
```

Previously, Appraisal would fail with `wrong number of arguments (given
0, expected 1) (ArgumentError)`

```
…/appraisal-2.5.0/lib/appraisal/bundler_dsl.rb:66:in `ruby': wrong number of arguments (given 0, expected 1) (ArgumentError)
	from …/appraisal-2.5.0/lib/appraisal/gemfile.rb:27:in `run'
	from …/appraisal-2.5.0/lib/appraisal/gemfile.rb:19:in `instance_eval'
	from …/appraisal-2.5.0/lib/appraisal/gemfile.rb:19:in `run'
	from …/appraisal-2.5.0/lib/appraisal/gemfile.rb:25:in `block in dup'
…
```

Author: swelther

lib/appraisal/bundler_dsl.rb

@@ -65,8 +65,9 @@ def source(source, &block)
       end
     end
 
-    def ruby(ruby_version)
+    def ruby(ruby_version = nil, **kwargs)
       @ruby_version = ruby_version
+      @ruby_version = kwargs unless ruby_version
     end
 
     def git(source, options = {}, &block)

spec/appraisal/gemfile_spec.rb

@@ -440,4 +440,17 @@
       expect(gemfile.load(tmpfile.path)).to include(File.dirname(tmpfile.path))
     end
   end
+
+  it "supports the ruby file: syntax" do
+    gemfile = Appraisal::Gemfile.new
+
+    gemfile.ruby file: ".ruby-version"
+    gemfile.source "one"
+
+    expect(gemfile.to_s).to eq <<-GEMFILE.strip_heredoc.strip
+      source "one"
+
+      ruby {:file=>".ruby-version"}
+    GEMFILE
+  end
 end