Fix CI build on AppVeyor by using Node 6
Fix CI build on AppVeyor by using Node 6

Seems a dependency (fs-extra) was using newer JS syntax that rendered it
incompatible with Node 4.8.7. Upgrading to Node 6 fixes the issue.

Merge of PR #481 
fsateler/MoreLINQ,morelinq/MoreLINQ,ddpruitt/morelinq,morelinq/MoreLINQ,fsateler/MoreLINQ,ddpruitt/morelinq