travis: Switch build matrix from os to jobs

There are a couple ways to declare a build matrix in Travis CI, refer to
https://docs.travis-ci.com/user/build-matrix.

Since upcoming commits will add more ways to run tests, switch the build
matrix from simple OS variation to a more powerful list of jobs.

.travis.yml

@@ -1,5 +1,7 @@
-script: test/run
 language: shell
-os:
+
-  - linux
+jobs:
-  - osx
+  - script: test/run
+    os: linux
+  - script: test/run
+    os: osx