Fix ScssFilter failing when sass is not installed in /usr/bin/sass.
Fix whitespace.
Make build timestamp file have more restrictive permissions.

This file will not need to be 0777 in most deployment scenarios. World
readable covers the situation where the CLI user and webserver user are
not the same, or in the same group.

While this could be achieved with sticky bits, basic permissions are
more reliable and portable.

Fixes #205