Support all active Node.js release lines

[Qard]

The Object.values(...) function was not added until Node.js 7.x. Use Object.keys(...).map(...) until the Node.js 6.x LTS expires.

Node.js 10.x+ also deprecated the use of a string matcher for the assert.throws(...) function, now throwing ERR_AMBIGUOUS_ARGUMENT.

All active release lines have been added to the .travis.yml to ensure complete test coverage.

Fixes #51

[coveralls]

Pull Request Test Coverage Report for Build 225

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.458%

---

Totals
Change from base Build 222:	0.0%
Covered Lines:	741
Relevant Lines:	819

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 225

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.458%

---

Totals
Change from base Build 222:	0.0%
Covered Lines:	741
Relevant Lines:	819

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 227

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 88.458%

---

Totals
Change from base Build 222:	0.0%
Covered Lines:	741
Relevant Lines:	819

---

💛 - Coveralls

[fieldju]

LGTM

[Qard]

Thanks for the quick response! 🎉