Fix `:` usage in line comment before `@import`

There is a similar test for this case here:
https://github.com/LearnBoost/stylus/commit/53c2879887c5f3d0757c1296e2133132fd15e41e#L2R1

But it will break if you insert some code before the first comment like this:
```
_ie = false
// should still import even with a comment ending in a colon:
@import "./import.basic/c"

// or any special character,
@import "./import.complex/c"
```