[bug fix] yarn.lock: 升级watchpack 版本，避免 node v10+报错

[zehuiguan]

Fixes #2250

Changes you made in this pull request:

• 升级watchpack 版本

[zehuiguan]

看来。。。travis 也得要设置 node10环境

[codecov-io]

Codecov Report

Merging #2251 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #2251   +/-   ##
=======================================
  Coverage   87.18%   87.18%           
=======================================
  Files         100      100           
  Lines        1772     1772           
  Branches      361      361           
=======================================
  Hits         1545     1545           
  Misses        177      177           
  Partials       50       50

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[chenjiahan]

感谢修正~ 后续会考虑把 travis 设置到 node 10 环境