Fix mxe apt repo selection.

Default to latest, which is bionic, for earlier repositories, match all
intermediate release code names.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>

installdeps

@@ -360,15 +360,15 @@ debian_installdeps() {
         case "$debian_rel" in
             bionic|stretch|trusty|xenial)
                 ;;
-            buster|cosmic|disco)
-                debian_rel=bionic
-                ;;
             yakkety|zesty|artful)
                 debian_rel=xenial
                 ;;
-            *)
+            utopic|vivid|wily)
                 debian_rel=trusty
                 ;;
+            *)
+                debian_rel=bionic
+                ;;
         esac
 
         mxe_apt_sources=/etc/apt/sources.list.d/mxeapt.list