sassProperty regex is very slow (freezes vim) with very long string values

using syntime I found that the following rule to be causing vim to completely freeze up on extremely long string values (the following example with 3s was with a much smaller string)


```
TOTAL      COUNT  MATCH   SLOWEST     AVERAGE   NAME               PATTERN
3.180625   26     0       3.177691    0.122332  sassProperty       \%([{};]\s*\|^\)\@<=\%([[:alnum:]-]\|#{[^{}]*}\)\+\s*:
```