Use two build jobs to speed up Travis CI.

Change-Id: I8f4dc30c4bace3c2a95c72b3fdd8b4eb200c80cd
Reviewed-on: https://swiftshader-review.googlesource.com/c/22249
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
diff --git a/.travis.yml b/.travis.yml
index 6848734..361d8e9 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -21,7 +21,7 @@
   - $CXX --version
   - mkdir build && cd build
   - cmake -DREACTOR_BACKEND=$REACTOR_BACKEND ..
-  - make
+  - make -j2
   - ./unittests
   - ./ReactorUnitTests