Add package-lock.json to gitignore

[IanEdington]

I accidentally added package-lock to a commit. Adding it to the gitignore to help future devs.

[coveralls]

Pull Request Test Coverage Report for Build 233

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 72.948%

---

Totals
Change from base Build 230:	0.0%
Covered Lines:	172
Relevant Lines:	236

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 233

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 72.948%

---

Totals
Change from base Build 230:	0.0%
Covered Lines:	172
Relevant Lines:	236

---

💛 - Coveralls

[gajus]

:tada: This PR is included in version 6.0.7 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: