fixed Pry::Editor#invoke_editor

Pry::Editor doesn't have a class method called invoke_editor. Fixed to call on an initialized Pry::Editor instance
gem install fails when config doesn't have --user-install set
Merge pull request #1306 from ahsandar/master

gem install fails when config doesn't have --user-install set
Merge pull request #1298 from dsisnero/patch-1

fixed Pry::Editor#invoke_editor
add 'gem-readme' command.