[beta] improves ci + coveralls

[huafu]

• AppVeyor
  • x86 => x64
  • node v 8 only instead of 6, 8 and 10
  • fails directly as soon as one of the node version's build fails
  • disable coverage
• Travis
  • node v 6 and 8 instead of 6, 8 and 10
  • enable coverage only on node v10
  • reports coverages to coveralls
  • improves commit filter

For reference, jest tests runs on Travis 10 and AppVeyor 8 only

It also adds this to PRs:

...and fails if the %age lose 1 or more point.