fix crash from TypeError in compass support

Using django-compressor with Compass I do
```scss
@import "compass/reset";
```
which gives me a big fancy error message
```python
SassEvaluationError: Error evaluating expression: elements-of-type(html5-block)
```
eventually ending in
```
.virtualenvs/project/lib/python2.7/site-packages/scss/types.py", line 563, in __init__ raise TypeError("Expected a Sass type, got %r" % (item,)) TypeError: Expected a Sass type, got 'article'
```

Drilling into the source code a bit shows that we are passing a list of normal python strings to the ``List`` constructor and it expects a list of ``Value``s. All I'm doing is converting all of those CSS elements into ``String``s before they get passed to the constructor which fixes the issue for me.