Fix problem compiling warn and debug statements in sass2css

See https://github.com/sass/libsass/issues/599

Source code:
````
@function str-replace($string, $old, $new: "", $case-sensitive: true)
	@if $string
		@warn "One of the 3 arguments is not a string"
		@return $string

	@return $string
````

Before patch:
````
@function str-replace($string, $old, $new: "", $case-sensitive: true) {
	@if $string {
		@warn "One of the 3 arguments is not a string" {}
		@return $string; }

	@return $string; }
````

After patch:
````
@function str-replace($string, $old, $new: "", $case-sensitive: true) {
	@if $string {
		@warn "One of the 3 arguments is not a string";
		@return $string; }

	@return $string; }
````