Relax Node dependency

[KiaraGrouwstra]

Bug Report

To reproduce: try installing Covalent on a new Node version (e.g. 6.9) through a package manager enforcing strict checks, such as Yarn.
This errors due to the "node": ">4.4 < 5", check in package.json. In actuality, a new Node version would work fine (as is the case when installing using npm over yarn).

What is the expected behavior?

Install without errors; a version check updated to match more recent versions that do in fact work.

What is the motivation / use case for changing the behavior?

Yarn is more desirable than NPM since it helps locking down exact package versions.

[emoralesb05]

Thanks for pointing us this issue, we are gonna lax our node dependency to < 7 for now, and maybe even remove the upper limit later on.

[emoralesb05]

Done with #136