Add diffutils to travis.yml and install_dependencies

It has been required for a while now, and now actually checked in
the makefiles. Before, if you didn't have it installed it would
just recompile everything.

The readme hasn't been updated to reflect this, I think we need
to go through that separately, and see what's really needed. Or
just instruct people to run the batch scripts.

.travis.yml

@@ -21,4 +21,5 @@ addons:
     - gcc-arm-none-eabi
     - binutils-arm-none-eabi
     - libnewlib-arm-none-eabi
+    - diffutils
 after_success: bash util/travis_compiled_push.sh