npm test is failing on older NodeJs versions due to `const` declaration

[mbernier]

It seems like the older versions of node doesn't know what to do with const variables during testing in travis.

Travis reporting failures

[mgingras]

@mbernier Looks like mocha drops support for 10.x as of their 4.0 release https://github.com/mochajs/mocha/releases . Freezing dev dependencies should keep tests passing. (Creating pr)

[gr8shivam]

As the project dependencies have dropped support for older versions of Node, tests for these versions will always fail.

Created PR #37 to remove older versions of node and add newer versions of node in travis config.