Loosen restriction on supported phantomjs-prebuilt versions

[bostaunieux]

I'm attempting to convert a project, which relies on this package via grunt-grunticon, to use yarn and there are known issues with yarn and phantomjs-prebuilt version 2.1.7 (yarnpkg/yarn#1538).

I've been able to modify yarn's configuration to force this project to use phantomjs-prebuilt 2.1.14, but the edits are a mess.

Ideally, this project would loosen it's restriction to support a range of phantomjs-prebuilt versions, so that manual override wouldn't be necessary.

Sample semver range that mirrors what svg-to-png uses:

  "phantomjs-prebuilt": "^2.1.7"

[jefflembeck]

I agree wholeheartedly. I think this may have been an oversight on our behalf.