ci(travis): add Node 14

[curbengh]

Node 13 to be deprecated in two months.

temporarily re-added back for debugging, ref hexojs/hexo#4260.

[coveralls]

Coverage remained the same at 96.321% when pulling 0d8ec66 on curbengh:node-14 into 936ef7d on hexojs:master.

[stevenjoezhang]

Confirmed that fs.promises.copyFile behave differently in Node.js 13 and 14

StevenJoeZhang@Mimis-Mac ~ % node
Welcome to Node.js v13.13.0.
Type ".help" for more information.
> fs.promises.copyFile('./test.txt', './test2.txt', {})
Promise { <pending> }
> 

StevenJoeZhang@Mimis-Mac ~ % node        
Welcome to Node.js v14.0.0.
Type ".help" for more information.
> fs.promises.copyFile('./test.txt', './test2.txt', {})
Promise {
  <rejected> TypeError [ERR_INVALID_ARG_TYPE]: The "mode" argument must be integer. Received an instance of Object
      at Object.copyFile (internal/fs/promises.js:211:10)
      at repl:1:13
      at Script.runInThisContext (vm.js:131:20)
      at REPLServer.defaultEval (repl.js:436:29)
      at bound (domain.js:429:14)
      at REPLServer.runBound [as eval] (domain.js:442:12)
      at REPLServer.onLine (repl.js:763:10)
      at REPLServer.emit (events.js:327:22)
      at REPLServer.EventEmitter.emit (domain.js:485:12)
      at REPLServer.Interface._onLine (readline.js:337:10) {
    code: 'ERR_INVALID_ARG_TYPE'
  }
}
> (node:21528) UnhandledPromiseRejectionWarning: TypeError [ERR_INVALID_ARG_TYPE]: The "mode" argument must be integer. Received an instance of Object
    at Object.copyFile (internal/fs/promises.js:211:10)
    at repl:1:13
    at Script.runInThisContext (vm.js:131:20)
    at REPLServer.defaultEval (repl.js:436:29)
    at bound (domain.js:429:14)
    at REPLServer.runBound [as eval] (domain.js:442:12)
    at REPLServer.onLine (repl.js:763:10)
    at REPLServer.emit (events.js:327:22)
    at REPLServer.EventEmitter.emit (domain.js:485:12)
    at REPLServer.Interface._onLine (readline.js:337:10)
(Use `node --trace-warnings ...` to show where the warning was created)
(node:21528) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 1)
(node:21528) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.

https://nodejs.org/dist/latest-v13.x/docs/api/fs.html#fs_fspromises_copyfile_src_dest_mode
https://nodejs.org/dist/latest-v14.x/docs/api/fs.html#fs_fspromises_copyfile_src_dest_mode

[stevenjoezhang]

Line 211 of the two files below shows the cause of the error. Node.js version 14.0 uses strict parameter type detection.
https://github.com/nodejs/node/blob/v14.0.0/lib/internal/fs/promises.js#L208-L216
https://github.com/nodejs/node/blob/v13.13.0/lib/internal/fs/promises.js#L208-L215

Relevant commit:
nodejs/node@a13500f#diff-0034beae2e92ed0b59570df638dda305

[curbengh]

#60