Add greenkeeper-lockfile devDependency

Although I am not currently using greenkeeper-lockfile to update the
package-lock.json file (since I currently prefer not to use
package-lock.json for CI), the dependency is required for the
Greenkeeper service to run builds for in-range updates.

See: https://blog.greenkeeper.io/were-changing-the-default-behaviour-for-lockfiles-ba9cb38a985a

Signed-off-by: Kevin Locke <[email protected]>

Author: kevinoid

package-lock.json

@@ -29,7 +29,7 @@
     "lint-js": "eslint . && echo ESLint passed.",
     "postpublish": "git -C doc push && git push --follow-tags origin master gh-pages && echo Remember to update GitHub Releases from CHANGELOG.md && echo until skywinder/github-changelog-generator#56 is fixed.",
     "postversion": "rimraf doc && git clone -b gh-pages -l -q . doc && npm run doc && git -C doc add . && git -C doc commit -n -m \"Docs for v$npm_package_version\"",
-    "preversion": "depcheck --ignores eslint-plugin-import --ignore-dirs doc && david && git-branch-is master && travis-status -b master -c -wx && appveyor-status -b master -c -w && istanbul check-coverage --statements 95 coverage/coverage.json",
+    "preversion": "depcheck --ignores eslint-plugin-import,greenkeeper-lockfile --ignore-dirs doc && david && git-branch-is master && travis-status -b master -c -wx && appveyor-status -b master -c -w && istanbul check-coverage --statements 95 coverage/coverage.json",
     "start": "supervisor --quiet --no-restart-on exit --extensions js,json --ignore test --poll-interval 2000 server.js",
     "test": "npm run lint && npm run test-unit",
     "test-cov": "npm run lint && npm run test-unit-cov",
@@ -46,6 +46,7 @@
     "eslint": "^4.6.1",
     "eslint-config-airbnb-base": "^12.0.0",
     "eslint-plugin-import": "^2.7.0",
+    "greenkeeper-lockfile": "^1.15.1",
     "istanbul": "^0.4.1",
     "jsdoc": "^3.4.1",
     "mocha": "^5.0.0",