CI should impose stricter standers on commits containing version bumps

[sjelin]

As we've discussed in person, e2e android CI tests are impossible on circle and travis. Still, here's a proposal for requiring npm test be run on version bumps:

1. We can check if a PR/commit is a version bump using

   // This is pseudo code, don't take it too literally
   exec('npm view . version') === exec('npm view webdriver-manager version')

   This is more reliable than diffing require('package.json').version from HEAD and HEAD^
2. If a commit is a version bump, npm test (and related CI commands) should ensure the following:
   • We are only one commit ahead of master:

     exec('git rev-parse HEAD^') ===
         exec('git ls-remote -q https://github.com/angular/webdriver-manager.git HEAD').split(' ')[0]

   • git diff-tree --no-commit-id --name-only -r HEAD shows the only files changed are package.json, CHANGELOG.md, and npm-test-history.json (more on that last file in a second), and package.json has only had its version changed
3. Whenever npm test passes, we do the following:

   require('npm-test-history.json')[os.type()] = currentHash;

   Where currentHash is git rev-parse HEAD if this isn't a version bump, git rev-parse HEAD^ if this is
4. On CI, if the PR is a version bump, in addition to (2), we make sure all the entries in npm-test-history.json equal git rev-parse HEAD^

This wouldn't be as good as real android CI, but it would stop us from releasing any bugs. It would also provide a clear log of the last time npm test was run on each OS