Bump levelgraph dependency to ^2.0.0

[BigBlueHat]

Fix #20

[mcollina]

CI is failing.

[BigBlueHat]

All three look like Travis mess ups (afaict).

For instance 0.12 and 4.0 are throwing this error (only finding levelgraph@1.3.1):
https://travis-ci.org/mcollina/levelgraph-n3/jobs/191365305#L179

The 0.10 run failed trying to make leveldown...
https://travis-ci.org/mcollina/levelgraph-n3/jobs/191365303#L230

Strangeness.

[mcollina]

Can you remove node v0.10 and v0.12 from .travis.yml? We just need to test on 4, 6, and 7.

[BigBlueHat]

Sorry for the wait @mcollina. Here's where Travis is unhappy:
https://travis-ci.org/mcollina/levelgraph-n3/jobs/195687992#L179

[mcollina]

@BigBlueHat you should replace level-test with memdb
https://github.com/BigBlueHat/levelgraph-n3/blob/5aa5fe7bbde33de18a428baf84ee9d2678f66913/package.json#L49

That was the same problem we had for levelgraph-jsonld.

[BigBlueHat]

@mcollina sorry for the wait! ⌚️