Workaround for Google Cloud SDK packaging problem

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.

Author: tautschnig

.travis.yml

@@ -56,7 +56,7 @@ jobs:
               key_url: 'https://packages.cloud.google.com/apt/doc/apt-key.gpg'
           packages:
             - cmake
-            - google-cloud-sdk
+            - google-cloud-sdk=216.0.0-0
             - graphviz
       cache:
         directories: