Fix YARD error when parsing LiveScript lexer (#1541)

A single line if-statement in the LiveScript lexer causes an error when
YARD runs over the project. Replacing it with a ternary statement fixes
the problem.

lib/rouge/lexers/livescript.rb

@@ -196,7 +196,7 @@ def self.loop_control_keywords
         rule %r/[#][{]/, Str::Interpol, :interpolated_expression
         # without curly braces
         rule %r/(#)(#{id})/ do |m|
-          groups Str::Interpol, if self.class.builtins.include? m[2] then Name::Builtin else Name::Variable end
+          groups Str::Interpol, (self.class.builtins.include? m[2]) ? Name::Builtin : Name::Variable
         end
       end