Speed up "push" builds.

[dhermes]

Travis has two build types that we use: "push" and "pull request". On a "pull request" commit the TRAVIS_BRANCH env. var. tells us what we are diff-ing against and makes it easy to limit the build jobs based on the changed files.

In "push" builds, the best we have is TRAVIS_COMMIT_RANGE but this is not a very dependable value. I propose we rely instead on the GitHub API to determine the changed files.

1. Try our best to parse the PR number from the commit message (if this fails, just bail and run every single test)
2. Verify the PR number by checking if the TRAVIS_COMMIT (and the HEAD commit in our local checkout) matches the merge_commit_sha in the PR endpoint: https://api.github.com/repos/GoogleCloudPlatform/google-cloud-python/pulls/2450
3. Once we have the PR, hit the PR files endpoint to get a list of changed files, e.g.: https://api.github.com/repos/GoogleCloudPlatform/google-cloud-python/pulls/2461/files

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As an alternative to the first step, there is an "unpublished" API which helps associate a commit SHA with a PR. For example, see the parenthetical 2461 below:

which tells us that 4e8afa8 came as part of #2461. The actual API returns an SVG with the payload containing

          <li class="pull-request">(<a href="/GoogleCloudPlatform/google-cloud-python/pull/2461" title="Merged Pull Request: Clarify docs on last 5 minutes query">#2461</a>)</li>

The API simply requires the commit hash in TRAVIS_COMMIT (i.e. just the HEAD commit on our local checkout), e.g. https://github.com/GoogleCloudPlatform/google-cloud-python/branch_commits/4e8afa8c3877376d7e9a6df79e6a272ae1cdfee7

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@tseaver @jonparrott PTAL and LMKWYT

FYI this will build on top of work done for #2277.

[dhermes]

Also worth noting that even if the files endpoint didn't exist, the PR endpoint has a base key which tells what state the main branch was in before the PR was merged. For example, https://api.github.com/repos/GoogleCloudPlatform/google-cloud-python/pulls/2450 has

  "base": {
    "label": "GoogleCloudPlatform:master",
    "ref": "master",
    "sha": "042daf5f2db6fdcc5323cfbdbcabcbf241fa7278",
    ...