explicity interpolate the variable

in sass 3.4, this scss:

```scss
@include keyframesNew('myanimation') {
    0% {
        opacity: 1;
    }
    100% {
        opacity: 0;
    }
}
```
with the non-interpolated variable name results in: 

```css
@keyframes $name {
  0% {
    opacity: 0;
  }
  100% {
    opacity: 0;
  }
}
etc
```

In 3.3 and 3.4, interpolating results in:

```css
@keyframes myanimation {
  0% {
    opacity: 1;
  }
  100% {
    opacity: 0;
  }
}
etc
```

for a live example, toggle the sass version at http://sassmeister.com/gist/c5640aa83a2622881bd5