.travis.yml: Add lzop

We need lzop now in order to make some FIT images that use LZO
compression on the contents.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/.travis.yml b/.travis.yml
index 7b53b3d..6cad65f 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -27,6 +27,7 @@
     - rpm2cpio
     - wget
     - device-tree-compiler
+    - lzop
 
 install:
  # Clone uboot-test-hooks