Use NPM 6 for CI (#301)

Author: AlekseyMartynov

appveyor.yml

@@ -6,7 +6,7 @@ environment:
 
 install:
   - ps: Install-Product node 8
-  - npm i -g npm@5
+  - npm i -g npm@6
   - if defined RELEASE_KEY_SECRET (nuget install secure-file -ExcludeVersion -Verbosity quiet && secure-file\tools\secure-file -decrypt net\DevExtreme.AspNet.Data\release.snk.enc -secret %RELEASE_KEY_SECRET%)
   - nuget install -Verbosity quiet -ExcludeVersion -OutputDirectory . OpenCover
   - pip install -q codecov