sysutils/cfengine-mastgerfiles: Make cfengine-materfiles318 default

cfengine-materfiles318 is now default.

Drop cfengine-masterfiles315 from the selection list.

sysutils/cfengine-masterfiles/Makefile

-VERSIONS=		315 316 317
-CFENGINE_VERSION?=	317
+VERSIONS=		316 317 318
+CFENGINE_VERSION?=	318
 
 MASTERDIR=		${.CURDIR}/../cfengine-masterfiles${CFENGINE_VERSION}