docs(guide/directives): improve Protractor test for bindings

[Narretz]

This needs Protractor >= 1.3.0 to work.

Closes #9330

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We found a Contributor License Agreement for you (the sender of this pull request) and all commit authors, but as best as we can tell these commits were authored by someone else. If that's the case, please add them to this pull request and have them confirm that they're okay with these commits being contributed to Google. If we're mistaken and you did author these commits, just reply here to confirm.

[gkalpak]

We are already using protractor: '^2.1.0' (even v1.3.x is on protractor: '^2.0.0'), so it shouldn't be a problem.

LGTM

[Narretz]

Thanks. I as just checking the CI for completeness (e2e wouldn't run right locally)