Travis: Add squashfs-tools

So that the tests we now have for squashfs can run, add squashfs-tools
for mksquashfs.
Signed-off-by: NTom Rini <trini@konsulko.com>

.travis.yml

@@ -51,6 +51,7 @@ addons:
     - sbsigntool
     - fakeroot
     - mtd-utils
+    - squashfs-tools
 
 install:
  # Clone uboot-test-hooks