Fixes for Node version 6

[robertjd]

Fixes stormpath/express-stormpath#438

@typerandom so here's the deal: the cloneextend module was calling hasOwnProperty() directly on objects, and that is brittle. Why this became a problem in node v6 I'm not sure and I didn't dig into it. But someone did fix this issue here, by using a more robust method for checking an object for properties:

https://github.com/shimondoodkin/nodejs-clone-extend/pull/5/files

Using that commit does solve the problem in stormpath/express-stormpath#438. That library isn't maintained very well, e.g. master has breaking changes in it, and package.json has a bumped minor number, which isn't even in npm anyways. As such, I'm moving the author's code into this library so that we aren't depending on a fragile library.

Other changes in this PR:

• The "JSON syntax error" message text has changed in v6, so I had to update the test assertion accordingly
• Adding node v6 to travis

[coveralls]

Coverage decreased (-11.7%) to 77.062% when pulling affda40 on node-v6-fixes into 8c92697 on master.

[typerandom]

I have tested and reviewed this and I'm happy with the change. I don't like inlining that patched clone-extend like that. But it's really one of the better alternatives right now, so I'm ok with it. We've had quite a bit of issues with it, so another solution to avoid inlining would be to create a separate package and also add tests for it so that we know that the ways that we want to extend functions as we expect them to.