fix(templates): adds tests for getFiles and replaces Array.flat

[philnash]

#123 broke support in Node < 11 due to using Array#flat 🤦‍♂️

This adds tests to ensure the getFiles function works properly and replaces Array#flat with lodash.flatten.

Fixes #124.

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[dkundel]

Looks good. Super nit pick: the URLs you use in the test are inconsistent between using twilio-labs and philnash. But otherwise this looks good. Thanks for the quick fix. Can you squash and release a new version?

[philnash]

Squashed and released.