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[tomap]

remove nodejs 6 support

[tomap]

note that I did not managed to make this work.
There is still a failing test and I don't know how to fix it

related to hexojs/hexo#3508

[coveralls]

Coverage remained the same at 94.286% when pulling 57ab07a on tomap:dropNodeJs6 into 51721d5 on hexojs:master.

[coveralls]

Coverage remained the same at 94.286% when pulling 97057da on tomap:dropNodeJs6 into 51721d5 on hexojs:master.

[curbengh]

hexo-server/test/index.js

Line 153 in 518fe81

.expect(200, '')

Change it to

    .expect(200, {})

for compatibility with supertest@4.

also todo (for myself or anyone to submit a separate PR); replace istanbul with nyc (see hexojs/hexo#3570 hexojs/hexo-generator-sitemap#51).

[tomap]

@weyusi thank you. it worked!

[curbengh]

for appveyor, I suggest [8, 10, 12].
Currently LTS and Stable are 10. LTS usually refers to more than one versions and can lead to minor confusion.

[yoshinorin]

I think appveyor setting is even another PR is OK :)