Replace levelup+memdown with level-mem 3.0.1 and upgrade level-ws to 1.0.0

[vpulim]

See: ethereumjs/ethereumjs-client#53

The current versions of the levelup, memdown and level-ws modules are very old. This PR replaces these with modern and up-to-date versions. There were some minor breaking API changes with the new versions so some code had to be modified.

Also, the newer versions of the level modules only support node 6 and higher.

All tests are passing but I'm not sure what the best way to make sure other external dependencies aren't broken. Ideas?

[coveralls]

Coverage decreased (-0.03%) to 94.55% when pulling df69683 on level-upgrade into a0717ce on master.

[holgerd77]

Thanks for this great! What do you mean with "external dependencies are broken", can you give an example?

[holgerd77]

Added one commit here updating the Node versions for Travis.

Would it be a way to do a very cautious 3.0.0 release here so that people (we) have to explicitly update and have another look what versions of the DB they are eventually passing?

[vpulim]

Thanks for this great! What do you mean with "external dependencies are broken", can you give an example?

An example would be a project with a dependency on merkle-patricia-tree but supports node 4. If that project upgraded to the new version of merkle-patricia-tree, it would no longer support node 4 and could possible break things for users running node 4.

Added one commit here updating the Node versions for Travis.

Would it be a way to do a very cautious 3.0.0 release here so that people (we) have to explicitly update and have another look what versions of the DB they are eventually passing?

Yes, this is a good idea and will hopefully prevent surprises for upstream dependencies.

[holgerd77]

I think we can very well ignore Node 4 users (if there are still any), I would generally regard me as cautious with expecting from users to always use the latest and greatest stuff, but library versions which have fallen out of the official support cycle of the maintainer can very well be dropped.

[holgerd77]

Looks good.