travis: use multiple build stages

This change rewrites .travis.yml to use many build stages/jobs. This
allows our build to run faster, as almost all jobs run in containers.

Stage 1: Run on all pushes to all branches
  - Job Build: just runs "make" to make sure everything is OK

Stage 2: Run on all PRs and pushes to master
  - Job Lint: Makes sure dep, "make gen", "make format", and "make lint"
              are all happy.
  - Job Build 1: Make sure "go get" and "make" will work. This job will
                 later run unit tests.
  - Job Build 2: Same as Job Build 1, except with another go version.
  - Job Integeration: Run integration tests (needs sudo, so needs VM)

Stage 3: Run on releases (if other stages pass)
  - Job Deploy: Publishes amd64 binaries to GitHub.

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