Test fix - specify electron-mocha version.

[bruce-one]

electron-mocha dropped support for node 6 in their 8.0.0 release - https://github.com/jprichardson/electron-mocha/blob/master/CHANGELOG.md

(Just triggering the CI for now, apologies for any noise.)

[bruce-one]

This seems to fix a lot of tests, but Node 11 on OSX seems to be failing with a segfault?

[bruce-one]

(Actually, that failure kinda looks to be a gremlin? It failed in 10 once, then failed in 11. Will kick it off again.)

[bruce-one]

On the whole this looks to help a lot, but the sporadic failures do seem to be consistently in the Backup api tests on OSX...

eg:
https://travis-ci.org/mapbox/node-sqlite3/jobs/520594580
https://travis-ci.org/mapbox/node-sqlite3/jobs/520594579
https://travis-ci.org/mapbox/node-sqlite3/jobs/520601166
https://travis-ci.org/mapbox/node-sqlite3/jobs/520698450
https://travis-ci.org/mapbox/node-sqlite3/jobs/520698443
https://travis-ci.org/mapbox/node-sqlite3/jobs/520708877
https://travis-ci.org/mapbox/node-sqlite3/jobs/520708873

All these failures state there's a segfault around the can backup from temp to main test, and they're all on OSX, but not all on one version. Afaict these are the only real test failures?

Not sure if that means there's some not ideal code in that area? I'm running the test suite in a while loop, but no failure yet...

I don't have access to any OSX machines to test on unfortunately.

[kewde]

Thank you @bruce-one!

I've spotted the segmentation fault as well, I'll ping @paulfitz as he's the author of the backup API.
The backup api is still experimental.

[kewde]

Hmmm, oddly enough it sometimes also returns a different error. Not only the segmentation fault error but instead:

86116 Bus error: 10

cfr: https://travis-ci.org/mapbox/node-sqlite3/jobs/521197134#L3926

To the best of my knowledge, this occurs in the test cannot backup to a locked db.

[kewde]

Merging, as the CI errors are unrelated to this PR.