travis: fix dependencies

bc hasn't been needed since a982c5d74f

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

.travis.yml

@@ -4,7 +4,7 @@ addons:
   apt:
     packages:
       - yasm
-      - bc
+      - diffutils
 compiler:
   - clang
   - gcc