运行grunt-test的时候，出现错误
Running "test" task Running "clean:server" (clean) task >> 0 paths cleaned. Running "concurrent:test" (concurrent) task Running "postcss:dist" (postcss) task Running "browserSync:test" (browserSync) task Running "mocha:all" (mocha) task Testing: http://localhost:9001/index.html Warning: Cannot read property 'pid' of undefined Use --force to continue. Aborted due to warnings.
