separate travis into multiple jobs

Parallelize travis by splitting the integration and unit tests into
their own jobs. Make images remains as part of the integration tests.

.travis.yml

@@ -6,6 +6,12 @@ env:
 go:
   - "1.13.3"
 go_import_path: github.com/GoogleContainerTools/kaniko
+jobs:
+  include:
+    - name: unit-test
+      script:
+        - make test
+    - name: integration-test
       before_install:
         - curl -fsSL https://download.docker.com/linux/ubuntu/gpg | sudo apt-key add -
         - sudo add-apt-repository "deb [arch=amd64] https://download.docker.com/linux/ubuntu $(lsb_release -cs) stable"
@@ -14,6 +20,5 @@ before_install:
         - curl -LO https://storage.googleapis.com/container-diff/latest/container-diff-linux-amd64 && chmod +x container-diff-linux-amd64 && sudo mv container-diff-linux-amd64 /usr/local/bin/container-diff
         - docker run -d -p 5000:5000 --restart always --name registry registry:2
       script:
-  - make test
         - make integration-test
         - make images