Remove Node 6 test builds

[nyurik]

Also added node 10

[orangemug]

@nyurik thanks for the PR, but unfortunately I'm planning to remove travisCI as now we have CircleCI running all the tests.

For reference issues with travis were, that it was taking a long time to build (occasionally many hours). Also with CircleCI we get per commit builds, because they host our artifacts for us. This means we can share a build in the PR, like in issue #323

[nyurik]

@orangemug thanks! I wasn't sure what was causing the failed build status on the repo, thought it was travis. Thx for looking into it!

[orangemug]

It turns out it was 00b22eb causing the issue. I managed to merge a failing PR somewhere along the way.... ooops! Once #358 is merge should be passing again.

Note: #358 is failing because of Appveyor, but I'm assuming Appveyor is just having a funny 5 mins, as it's happened before and I don't have a window box to test on. I'll push another blank commit to force and build and get these tests passing again.