assets/makefile: use bash instead of sh

on _older_ linux distros (by older I mean not Fedora)
sh interpreted assets/generator.sh incorrectly:
echo -e wasn't interpreted as echo with -e switch

fix #3

assets/Makefile

@@ -1,4 +1,4 @@
 all: build
 
 build:
-	sh generate.sh
+	bash generate.sh