Don't parse static values in declarations

Ruby sass has some smarts around determining whether a declarations value needs to be parsed i.e. if it has function calls for interpolants. If the value looks static it's left untouched.

This behaviour was previously mentioned in https://github.com/sass/libsass/pull/658#issuecomment-63612597.

With this change we can finally address the `not` problem (#368) by re-applying 4e6ac82e588a72fa60c7ab65f1ec17ea68bd2876.