🏗 Synchronize exit and fix logging on Travis for dev dashboard tests

[rsimha]

PR highlights:

• Synchronizes gulp task exit to prevent the Did you forget to signal async completion? error
• Makes Travis logging silent when tests pass and concise when they fail

Before (tests fail):

[20:46:20] Using gulpfile ~/src/amphtml/gulpfile.js
[20:46:20] Starting 'dev-dashboard-tests'...


  ․․․․․․․․․․․․․․․․․․․․․․․․․․․․․!․․․․․․․․․․․․․․․․․․

  47 passing (473ms)
  1 failing

  1) devdash
       Test helpers
         parseHtmlChunk
           returns firstElementChild:

      AssertionError: expected 2 to equal 3
      + expected - actual

      -2
      +3
      
      at Context.it (build-system/app-index/test/test-self.js:38:22)
      at process.topLevelDomainCallback (domain.js:126:23)



[20:44:23] The following tasks did not complete: dev-dashboard-tests
[20:44:23] Did you forget to signal async completion?

Before (tests pass):

[20:56:23] Using gulpfile ~/src/amphtml/gulpfile.js
[20:56:23] Starting 'dev-dashboard-tests'...


  ․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․․

  48 passing (454ms)

[20:56:24] Finished 'dev-dashboard-tests' after 1.13 s

After (tests fail):

[20:47:34] Using gulpfile ~/src/amphtml/gulpfile.js
[20:47:34] Starting 'dev-dashboard-tests'...
devdash Test helpers parseHtmlChunk returns firstElementChild (build-system/app-index/test/test-self.js)
AssertionError: expected 2 to equal 3
    at Context.it (build-system/app-index/test/test-self.js:38:22)
    at process.topLevelDomainCallback (domain.js:126:23)

 1 test failed
[20:47:35] Finished 'dev-dashboard-tests' after 1.11 s

After (tests pass):

[20:48:03] Using gulpfile ~/src/amphtml/gulpfile.js
[20:48:03] Starting 'dev-dashboard-tests'...
[20:48:04] Finished 'dev-dashboard-tests' after 1.11 s