travis: Refactor jobs to use inheritance

Since the existing jobs are similar (currently, the `script` keys are
same), move the equal parts to a new shared key to have them in one
place. The new map will receive more content in upcoming commits.

.travis.yml

@@ -1,7 +1,15 @@
+# YAML anchors need to appear first.
+# Keys starting with an underscore are the custom ones, refer to
+# https://docs.travis-ci.com/user/build-config-yaml#private-keys-as-yaml-anchors-and-aliases-and-external-tooling
+
+_native_job: &native_job
+  script: |
+    test/run
+
 language: shell
 
 jobs:
-  - script: test/run
+  - <<: *native_job
     os: linux
-  - script: test/run
+  - <<: *native_job
     os: osx