Fix indentation with interpolated strings in ruby 1.9.3

Commit: 2cc5fcb6762e5f3ca36e89be96fb7b6d274f6672 works in ruby 2.0.0, but it doesn't work in ruby 1.9.3,
because in Ripper in ruby 1.9.3, the right brace of an embedded expression invokes `on_rbrace`,
while in ruby 2.0.0 it invokes `on_embexpr_end`.