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annotator: Fix installation of wicked-good-xpath

This was testing the type of the wrong object, and thus
wicked-good-xpath wasn't being correctly installed.
Install wgxpath shim for IE9+ in the test runner

This commit adapts the work done in opengovfoundation/annotator-ie-shims
to make our tests pass in IE9+.

Left as an exercise for the future:

- including the shim in a built package
- IE8
Don't permit test failures in IE9/10/11

As the tests now pass in these browsers, we won't allow regressions.
Replace chai with assertive-chai

IE8 can't deal with the use of ES5 in the chai library. Luckily, the
relevant part of chai is used only by the expect/should assertion
format, which we don't use. assertive-chai includes only the assert API
as a wrapper around Node's assert package, which has a corresponding
browserify shim.