fix(sourcemaps): allow for rewriting sourceMappingURL in compressed output

This PR allows for external sourcemap support while using the `compressed` output style. libsass does not add a newline prior to outputting the `sourceMappingURL` in this mode. Previously, the regexp would fail due to specificity and would result in two `sourceMappingURL` entries. The incorrect (first) one would then be parsed by downtstream processors, breaking sourcemaps.