Workaround for Google Cloud SDK packaging problem

[tautschnig]

Version 217.0.0-0 of the google-cloud-sdk package using XZ compression,
which is not supported by the version of dpkg available on Ubuntu
Trusty. This change fixes the package to the prior version, which should
remain in place until https://issuetracker.google.com/issues/116076881
has been fixed.

• Each commit message has a non-empty body, explaining why the change was made.
• My contribution is formatted in line with CODING_STANDARD.md.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[peterschrammel]

LGTM assuming that CI passes

[allredj]

This PR failed Diffblue compatibility checks (cbmc commit: 73d7348).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/85252499
Status will be re-evaluated on next push.
Please contact @peterschrammel, @thk123, or @allredj for support.

Common spurious failures:

• the cbmc commit has disappeared in the mean time (e.g. in a force-push)
• the author is not in the list of contributors (e.g. first-time contributors).

[allredj]

Passed Diffblue compatibility checks (cbmc commit: 60540c7).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/85253364