[Fix #1000] Support modifier and def on same line in IndentationWidth

Enforce an indentation style for Ruby >= 2.1 when "private def" etc
is used.
[Refactor] Shorten IndentationWidth

Needed to make it pass rubocop inspection with ClassLength/Max 121.
Merge pull request #1007 from jonas054/fix_1000_indent_after_private_def

Fix indent after private def