Bump libsass up a commit to fix compilation of interpolated placeholders

hcatlin/libsass@5660bfe fixes an issue I came across while working with Inuit.css:

```
$inuit-namespace: null !default;
[class~="#{$inuit-namespace}1/2"]
```
is rendered as 
```
[class~="#{$inuit-namespace}1/2"]
```
when it should be rendered as
```
[class~="1/2"]
```