feat(umi-build-dev): disable mock via MOCK=none

[xiaohuoni]

$ MOCK=none umi dev
Respond #486

[sorrycc]

应该改在 packages/umi-build-dev/src/getPlugins.js。

[coveralls]

Pull Request Test Coverage Report for Build 436

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.4%) to 20.686%

---

Files with Coverage Reduction	New Missed Lines	%
packages/umi-build-dev/src/getPlugins.js	1	0.0%
packages/umi-plugin-dva/src/index.js	3	27.39%

Totals
Change from base Build 430:	0.4%
Covered Lines:	462
Relevant Lines:	2470

---

💛 - Coveralls

[xiaohuoni]

改成这样，感觉怪怪的

const builtInPlugins = [
    './plugins/output-path',
    './plugins/global-js',
    './plugins/global-css',
    './plugins/antd',
    './plugins/fastclick',
    './plugins/hd',
    './plugins/hash-history',
    './plugins/afwebpack-config',
    './plugins/404', // 404 must after mock
  ];
  if(MOCK){
    builtInPlugins.push('./plugins/mock')
  }

[sorrycc]

mock 有顺序关系的，不能换位置，应该这样：

- './plugins/mock',
+ ...(process.env.MOCK === 'none' ? [] : ['./plugins/mock']),