travis: Clone official images in to working directory

* Don't pollute the home directory
* Ignore the local git repo for sanity's sake.

Author: kylemanna

.gitignore

@@ -0,0 +1 @@
+/official-images

.travis.yml

@@ -5,15 +5,15 @@ env:
   - VARIANT=trusty
 
 install:
-  - git clone https://github.com/docker-library/official-images.git ~/official-images
+  - git clone https://github.com/docker-library/official-images.git official-images
 
 before_script:
   - env | sort
   - image="bitcoind:$VARIANT"
 
 script:
   - docker build -t "$image" .
-  - ~/official-images/test/run.sh "$image"
+  - official-images/test/run.sh "$image"
 
 after_script:
   - docker images