fix multi target for webpack-dev-server task

[ljqx]

Q	A
Bug fix?	yes
Breaking change?	no
New feature?	no
Deprecations?	no
Tests added?	no
Docs updated?	no
Fixed tickets	none
License	MIT

This PR is counterpart of #140 which fix multi target for webpack task.

This is a bug when running grunt webpack-dev-server with multi targets. If we define multi targets for this plugin, then only 1 or 2 of them will run when user calls grunt webpack-dev-server, and the tasks are picked randomly.

This is because now the code use a global done for all targets, any of them finished will cause this task done.

[codecov]

Codecov Report

Merging #143 into master will decrease coverage by 1.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
- Coverage   67.67%   66.66%   -1.02%     
==========================================
  Files           8        8              
  Lines         198      201       +3     
  Branches       50       51       +1     
==========================================
  Hits          134      134              
- Misses         52       55       +3     
  Partials       12       12

Impacted Files	Coverage Δ
tasks/webpack-dev-server.js	15.55% <0%> (-1.12%)	⬇️

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