[travis] revamp multi-platform build

[luclu]

Needs rebase on Taylor's merged PR

fixes #1182

Currently splits the builds in (1: sudo trusty) linux + win and (2: osx) mac.

Outstanding TODO:

• container-based instead of full-virt
  • use addons: apt:, travis whitelist "PRs":
    • APT whitelist request for graphicsmagick in TRUSTY travis-ci/apt-package-safelist#3765
    • APT whitelist request for gcc-multilib in TRUSTY travis-ci/apt-package-safelist#3766
      • APT source whitelist request for gcc-multilib travis-ci/apt-source-safelist#339
    • APT whitelist request for icnsutils in TRUSTY travis-ci/apt-package-safelist#3764
• osx build: certain steps take way too long, error out:
  • copy-app-folder-files: 1.48 min instead of 3.91 s (trusty)
  • download-signatures: errored after 1.52 min instead of 4.49 s (trusty)

Selected platform: [ 'mac' ]
[14:01:05] Using gulpfile ~/build/ethereum/mist/gulpfile.js
[14:01:05] Starting 'mist'...
[14:01:05] Starting 'set-variables-mist'...
[14:01:05] Finished 'set-variables-mist' after 140 μs
[14:01:05] Starting 'download-signatures'...
[14:01:05] Starting 'clean:dist'...
[14:01:05] Finished 'clean:dist' after 28 ms
[14:01:05] Starting 'copy-app-source-files'...
[14:01:05] Starting 'copy-node-folder-files'...
[14:01:06] Finished 'copy-app-source-files' after 1.62 s
[14:01:06] Starting 'copy-app-folder-files'...
[14:03:35] Finished 'copy-app-folder-files' after 2.47 min
[14:03:35] Starting 'copy-build-folder-files'...
[14:03:35] 'download-signatures' errored after 2.5 min
[14:03:35] ParseError
    at stream.catch.then.data (/Users/travis/build/ethereum/mist/node_modules/got/index.js:114:14)
    at process._tickCallback (internal/process/next_tick.js:103:7)
[14:03:35] 'mist' errored after 2.5 min
[14:03:35] ParseError in plugin 'run-sequence(download-signatures)'
Message:
    Unexpected end of JSON input in "https://www.4byte.directory/api/v1/signatures/?page_size=20000&ordering=created_at": 
{"count":3337,"next":null,"previous":null,"results":[{"id":112,"created_at":"...
Details:
    host: www.4byte.directory
    hostname: www.4byte.directory
    method: GET
    path: /api/v1/signatures/?page_size=20000&ordering=created_at
    statusCode: 200
    statusMessage: OK
Stack:
ParseError
    at stream.catch.then.data (/Users/travis/build/ethereum/mist/node_modules/got/index.js:114:14)
    at process._tickCallback (internal/process/next_tick.js:103:7)
[14:03:35] Finished 'copy-build-folder-files' after 334 ms
[14:03:35] Finished 'copy-node-folder-files' after 2.5 min
/Users/travis/build.sh: line 151:  3660 Terminated: 15          travis_jigger $! $timeout $cmd

• upload to github releases if tag available
• promisify download-signatures as the command does not finish in time for the copy stage when using travis, simpler alternative: use sync instead of async but will increase build-time by 3-7s
• no continuous output of travis wait-wrapped command travis_wait discards wrapped command output travis-ci/travis-ci#5716

[mention-bot]

@luclu, thanks for your PR! By analyzing the history of the files in this pull request, we identified @hiddentao to be a potential reviewer.

[luclu]

superseeded