Ensure #dump doesn't add a new line in config

Fixes #2821

* YAML#dump without any option parameters hard-wraps a string at 80
  characters and this causes errors while reading the config file
  and parsing it.

* There is another way to fix this: Modify the Regex that is used for
  parsing the config file. The current regex supports only a single line
  of "key: value" type statements. Presumably, YAML#load is not used for
  security reasons and so, this is the only other option.

	modified:   lib/bundler/settings.rb
	modified:   spec/commands/config_spec.rb