Run `make setup` in Travis with `-j8`.

Previously, QEMU's build in Travis was run with -j8. Now QEMU is built by `make setup`, so we should pass `-j8` to `make setup` instead.

.travis.yml

diff --git a/.travis.yml b/.travis.yml
index b73b4cc..52e5d38 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -14,7 +14,7 @@
 cache: cargo
 
 install:
-  - make setup
+  - make -j8 setup
 
 script:
   - make test