build: transition to circle 2 and cache test fixtures in CI

[kjin]

This change migrates Linux-based CI completely to CircleCI (2.0), and adds caching of node_modules (invalidated when either package-lock.json or package.json change) and build/test/plugins/fixtures (invalidated when test/fixtures/plugin-fixtures.json changes) to both CircleCI and AppVeyor. This results in a 5-8 min speedup for each test run across CircleCI and AppVeyor.

delete all plugin fixtures

• Removes all directories in test/plugins/fixtures in favor of plugin-fixtures.json
  • This is beneficial because CIs generally invalidate caches when a certain file is changed. Coalescing all the information found in the plugin fixtures makes this a lot easier.

fix CI failures

• Fixes two CI failures introduced with:
  • Node v9
  • TypeScript v???

modify scripts

• Names exported functions in scripts rather than using export default
• Removes some pre-requisite steps in npm scripts for atomicity
• Adds encrypt-service-account-credentials script to regenerate encrypted system test credentials (for future use; this is meant to be used manually and infrequently)
• Modifies init-test-fixtures script to generate plugin fixtures based off plugin-fixtures.json file
• Adds plugin-fixtures.json
• Changes code coverage reporter to codecov

update system test key file

• As Travis (rightfully) doesn't allow me to access encryption details for system test credentials, this commit is the result of running encrypt-service-account-credentials to regenerate a new encrypted key file (new encryption details have been added to CircleCI as env vars)

update CI configurations

• Deletes .travis.yml and circle.yml
• Adds .circleci/config.yml (w/ caching)
• Modifies appveyor.yml to add caching and change list of commands to run

update package-lock.json

• Self-explanatory

[codecov]

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[DominicKramer]

LGTM