Revert "Conditionally disable some more GSL >= 1.15 code"

This reverts commit 1ca1503a08528482708ca864c171212a0dd46f96.
Revert "Disable GSL 1.15-only code if GSL is too old"

This reverts commit 51e5c5e30565537fbc4eafa2d94cd0810f1134bc.
LALInference now depends on GSL >= 1.15