Update PhantomJS unit tests

[jtpalmer]

Description

Replaces use of RawGit with unpkg. Changes the version of chai to 3.4.1 because version 3.4.2 is not available on npm. Also adds an onResourceError handler to improve debugging.

Motivation and Context

These unit tests were failing on Travis CI and RawGit has reached the end of its useful life.

Tests performed

Tested manually and on Travis CI.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project as found in the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[plessbd]

Any reason you picked unpkg over jsDelivr?

Really I have no opinion just wondering if there was any reasons.

Shippable will fail unti our cloud issues are resolved.

[jtpalmer]

I picked unpkg because I've seen it used somewhere. I don't remember where I saw it.

[plessbd]

as good of a reason as I would have had for using any of the others, so works for me.