Regression in parent reference interpolation in selector

[xzyfer]

Original report sass/node-sass#1365

.foo {
  &--#{'bar'} {
    color: red;
  }
}

LibSass 3.3.2

.foo--bar {
  color: red; }

LibSass 3.3.3

Error: Invalid CSS after ".foo {": expected "}", was "&--#{$bar} {"
        on line 3 of test.scss
>> .foo {

Spec added sass/sass-spec#726

[xzyfer]

Bisected the regression to 592ff47