Remove inner quotes from the "unexpected `"x"'" error message.
+ lexer.rl: disallow "$-" (dollar, dash, no character) special.

This commit tracks upstream Ruby 2.1 commit https://github.com/ruby/ruby/commit/531329a2.

Technically it introduces incompatibility for 2.0 and prior syntaxes,
but it is done this way to keep lexer simple. This could be fixed if
it causes problems in practice.