ENYO-455: Set explicit version of chai.

### Issue
When `chai` was automatically updated to version `1.10.0` from `1.9.2` in Travis, a number of assertions began throwing a TypeError

    TypeError: '0' is not a function (evaluating 'expect(foo).to.exist.and.to.have.length(bar)')

### Fix
Changing the assertion from the form

    expect(foo).to.exist.and.to.have.length(bar)

to the form

    expect(foo).to.exist().and.to.have.length(bar)

seems to fix the errors, but it's unclear exactly why. `1.10.0` introduced noop functional calls for assertion properties, so this seems potentially related (and was the inspiration for trying this change). While this is still being investigated, we explicitly set the version of `chai` to the last known good version, which is `1.9.2`.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>