Fix some stuff with travis

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

Author: cpuguy83

.travis.yml

@@ -5,4 +5,4 @@ services:
 
 script:
   - ./update.sh && git diff --quiet HEAD
-  - for i in $(git diff --stat --name-only | grep Dockerfile); do docker build -< $i; done
+  - for i in $(git diff --stat --name-only HEAD~1 | grep Dockerfile); do docker build -< $i; done