Revert "[wip/cppadcodegen] fix minimum cmake version"

This reverts commit 9a1be0dd.
wip/cppadcodegen needs 2.8+, but wip/cppad needs 3.1+.

cppad/Makefile

@@ -20,7 +20,7 @@ LICENSE=		eclipse
 MAINTAINER=		gsaurel@laas.fr
 
 CMAKE_ARGS+=	-Dcppad_prefix=${PREFIX}
-DEPEND_ABI.cmake=	cmake>=2.8
+DEPEND_ABI.cmake=	cmake>=3.1
 
 include ../../mk/sysdep/cmake.mk
 include ../../mk/language/c.mk