Fix testing by installing 7z

ranma42 · ranma42 · commit 7c7dec0ebada · 2017-04-11T16:00:20.000+02:00
provided by the `p7zip-full` package.
diff --git a/.travis.yml b/.travis.yml
@@ -1,2 +1,7 @@
+addons:
+  apt:
+    packages:
+    - p7zip-full
+
 script:
   - ./test.sh
