Fix compressed output of media queries

This PR fixes the output of `@media` blocks which broke in the 3.0.0 release. This PR simply makes the compressed output mimic the behaviour of nested output which doesn't suffer from this bug.

See: https://github.com/sass/libsass/blob/master/output_nested.cpp#L189-L192


Fixes https://github.com/sass/libsass/issues/566.

No spec was added because this requires compressed output. A naive test of this fix can be performed locally by running.

```bash
$ cd SASS_LIBSASS_PATH
$ make
$ cd SASS_SASSC_PATH
$ make
$ echo "@media screen and (min-width: 400px) {body {background-color: purple;}}" | sassc/bin/sassc -t compressed -s
// @media screen and (min-width: 400px){body{background-color:purple}}
```