Remove deprecated groups syntax

Note that `group` syntax is still available. This gets removed because
it is not officially supported by Bundler.

Fix #83

lib/appraisal/gemfile.rb

@@ -41,16 +41,6 @@ def group(*names, &block)
       @groups[names].run(&block)
     end
 
-    # :nodoc:
-    def groups(*names, &block)
-      $stderr.puts <<-WARNING.gsub(/\n\s+/, " ").strip
-        Warning: `#groups` is deprecated and will be removed in 2.0.0.
-        Please use `#group` instead.
-      WARNING
-
-      group(*names, &block)
-    end
-
     def platforms(*names, &block)
       @platforms[names] ||= Platform.new(names)
       @platforms[names].run(&block)

spec/appraisal/gemfile_spec.rb

@@ -52,24 +52,6 @@
     GEMFILE
   end
 
-  it 'supports groups syntax, but with deprecation warning' do
-    gemfile = Appraisal::Gemfile.new
-
-    warning = capture(:stderr) do
-      gemfile.groups :development, :test do
-        gem "one"
-      end
-    end
-
-    expect(gemfile.to_s).to eq <<-GEMFILE.strip_heredoc.strip
-      group :development, :test do
-        gem "one"
-      end
-    GEMFILE
-
-    expect(warning).to match(/deprecated/)
-  end
-
   it 'supports platform syntax' do
     gemfile = Appraisal::Gemfile.new