travis: Don't use -9 on gzip

I timed this locally and plain old `gzip` took 2m06s while `gzip -9` took a
whopping 6m23s to save a mere 4MB out of 1.2GB. Let's shave a few minutes off
the Android builder by turning down the compression level.

.travis.yml

@@ -76,7 +76,7 @@ before_cache:
   - docker history -q rust-ci |
     grep -v missing |
     xargs docker save |
-    gzip -9 > $HOME/docker/rust-ci.tar.gz
+    gzip > $HOME/docker/rust-ci.tar.gz
 before_install:
   - zcat $HOME/docker/rust-ci.tar.gz | docker load || true