Fix NoMethodError in backend (#364)

- IRB v1.15.0 renamed build_statement -> parse_input

lib/iruby/backend.rb

@@ -58,14 +58,16 @@ def eval_binding
     end
 
     def eval(code, store_history)
-      if Gem::Version.new(IRB::VERSION) < Gem::Version.new('1.13.0')
-        @irb.context.evaluate(code, 0)
-      else
-        @irb.context.evaluate(@irb.build_statement(code), 0)
-      end
+      @irb.context.evaluate(parse_code(code), 0)
       @irb.context.last_value unless IRuby.silent_assignment && assignment_expression?(code)
     end
 
+    def parse_code(code)
+      return code if Gem::Version.new(IRB::VERSION) < Gem::Version.new('1.13.0')
+      return @irb.parse_input(code) if @irb.respond_to?(:parse_input)
+      return @irb.build_statement(code) if @irb.respond_to?(:build_statement)
+    end
+
     def complete(code)
       if @completor
         # preposing and postposing never used, so they are empty, pass only target as code