fix(shelf): Replace expired test certificates

[brianquinlan]

Fixes #488

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[mosuem]

We could also add a regeneration script as in dart-lang/http#1849 - I am fine with either.

[kevmoo]

@brianquinlan – can haz land?

[brianquinlan]

@brianquinlan – can haz land?

@mosuem has a better approach. Something will land by tomorrow (either by myself or by @mosuem) ;-)