Add Ruby-2.6 detection.

Signed-off-by: gregory guy <gregory-tde@laposte.net>
(cherry picked from commit 046b21e2c2)

lib/kross/configure.in.in

@@ -36,7 +36,7 @@ if test "x$compile_kross" = "xyes" ; then
         RUBY_CFLAGS=`$PKGCONFIG ruby --cflags`
       fi
       if test -z "$RUBY_VERSION"; then
-        for RUBY_TEST in 2.5 2.4 2.3 2.2 2.1 2.0 1.9; do
+        for RUBY_TEST in 2.6 2.5 2.4 2.3 2.2 2.1 2.0 1.9; do
           RUBY_VERSION=`$PKG_CONFIG ruby-$RUBY_TEST --modversion 2>/dev/null`
           if test -n "$RUBY_VERSION"; then
             RUBY_CFLAGS=`$PKG_CONFIG ruby-$RUBY_TEST --cflags`