gulp: support gulp v4

[lvaccaro]

Gulp updates for v4.x

[RCasatta]

~/git/opentimestamps.org(gulp-fix)$ gulp --version
4.0.2
~/git/opentimestamps.org(gulp-fix)$ gulp build
Error: Cannot find module 'run-sequence'
Require stack:
- /home/casatta/git/opentimestamps.org/gulpfile.js
- /usr/share/nodejs/gulp-cli/lib/shared/require-or-import.js
- /usr/share/nodejs/gulp-cli/lib/versioned/^4.0.0/index.js
- /usr/share/nodejs/gulp-cli/index.js
- /usr/share/nodejs/gulp/bin/gulp.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:815:15)
    at Function.Module._load (internal/modules/cjs/loader.js:667:27)
    at Module.require (internal/modules/cjs/loader.js:887:19)
    at require (internal/modules/cjs/helpers.js:74:18)
    at Object.<anonymous> (/home/casatta/git/opentimestamps.org/gulpfile.js:16:21)
    at Module._compile (internal/modules/cjs/loader.js:999:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1027:10)
    at Module.load (internal/modules/cjs/loader.js:863:32)
    at Function.Module._load (internal/modules/cjs/loader.js:708:14)
    at Module.require (internal/modules/cjs/loader.js:887:19) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [
    '/home/casatta/git/opentimestamps.org/gulpfile.js',
    '/usr/share/nodejs/gulp-cli/lib/shared/require-or-import.js',
    '/usr/share/nodejs/gulp-cli/lib/versioned/^4.0.0/index.js',
    '/usr/share/nodejs/gulp-cli/index.js',
    '/usr/share/nodejs/gulp/bin/gulp.js'
  ]
}

[lvaccaro]

'run-sequence' should not be necessary anymore