Support the 0.19 version of google-cloud-trace

[brianquinlan]

google-cloud-trace is at version 0.19.0:
https://github.com/GoogleCloudPlatform/google-cloud-python/blob/master/trace/setup.py

We should support that version to prevent errors when installing google-cloud-trace and opencensus i.e.
$ pip install google-cloud-trace opencensus
...
opencensus 0.1.4 has requirement google-cloud-trace<0.18dev,>=0.17.0, but you'll have google-cloud-trace 0.19.0 which is incompatible.

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[brianquinlan]

I signed it!

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CLAs look good, thanks!