Update composer.lock to cloud-tools to v6.0

[bshaffer]

This can be done by running the following bash command:

find . -name composer.lock -not -path "*/vendor/*" \
    -exec sh -c 'pushd "${0%/*}" && composer update && popd' {} \;

[tmatsuo]

Also we'd like to update google/cloud to '0.20'.

find . -name composer.json -not -path "*/vendor/*" -exec sh -c 'pushd "${0%/*}" && grep \"google/cloud\" composer.json && composer require google/cloud:'~0.20' google/auth:'~0.11' && composer update && popd' {} \;

find . -name composer.json -not -path "*/vendor/*" -exec sh -c 'pushd "${0%/*}" && grep \"google/cloud-tools\" composer.json && composer require --dev google/cloud-tools:'~0.6' && composer update && popd' {} \; 

and I get #271