Bump rubocop to 0.20.1

https://trello.com/c/ECCr8yci/199-upgrade-rubocop

* Replace DefWithoutParentheses with MethodDefParentheses
* Replace ReduceArguments with SingleLineBlockParams
* Replace SpaceAroundBlockBraces with SpaceBeforeBlockBraces and SpaceInsideBlockBraces
* Remove SymbolName
* Replace config for DefWithParentheses with MethodDefParentheses
* Update DotPosition configuration to new style
* Remove SingleLineBlockParams
* Update RuboCop configuration