ci: use travis containers

[filipesilva]

The trusty env on travis now supports container based builds: https://docs.travis-ci.com/user/trusty-ci-environment/

This should reduce start-up time.

[filipesilva]

nvm, container based doesn't have yarn :(

[filipesilva]

Apparently containers have an old version of Chrome:

[02:57:53] E/launcher - unknown error: Chrome version must be >= 55.0.2883.0

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