Add Travis for Testing and Linting

[grant]

Summary

• Adds .travis.yml file with Node setup
  • Travis lints the project for every commit/PR. If there are lint errors, the Travis build will fail.
  • For this PR, I manually triggered a Travis job, which succeeded:
    • https://travis-ci.org/grant/apps-script-oauth2/builds/368327394
      • (Trigger a build in Travis with: More options > Trigger a Custom Build > master)
• Adds Travis build badge in README

Job Logs

[20:08:24] Using gulpfile ~/build/grant/apps-script-oauth2/gulpfile.js
[20:08:24] Starting 'lint'...
[20:08:25] Finished 'lint' after 1.02 s
The command "npm run lint" exited with 0.
0.89s$ npm run test
> apps-script-oauth2@1.25.0 test /home/travis/build/grant/apps-script-oauth2
> mocha
  Service
    #getToken()
      ✓ should return null when no token is stored
      ✓ should return null after the service is reset
      ✓ should load from the cache
      ✓ should load from the properties and set the cache
      ✓ should not hit the cache or properties on subsequent calls
    #saveToken_()
      ✓ should save the token to the properties and cache
    #reset()
      ✓ should delete the token from properties and cache
    #hasAccess()
      ✓ should use the lock to prevent concurrent access (105ms)
    #refresh()
      ✓ should use the lock to prevent race conditions (111ms)
  Utilities
    #extend_()
      ✓ should extend (left) an object
      ✓ should extend (right) an object
      ✓ should extend (merge) an object
  12 passing (234ms)
The command "npm run test" exited with 0.
cache.2
store build cache
0.01s
3.10snothing changed, not updating cache
Done. Your build exited with 0.

[erickoledadevrel]

Awesome! Surprisingly easy to setup, thanks for doing this.