Make $TRUE and $FALSE public

When extending lessphp with custom predicate functions, returning $less::$TRUE
or $less::$FALSE is both more concise and more readable than using literal
arrays -- and less brittle, too, since it allows the details of how tokens are
represented in lessphp to remain encapsulated in the compiler class.
Accordingly I've changed both $TRUE and $FALSE properties to be public rather
than protected.