Support TravisCI integration (closes #29)

[jwflory]

Review #32 before merging!

This commit does a few things:

• Adds a TravisCI configuration file, to run preCommitChecks.sh in npm test
• Adds a pretest, test script to package.json, uses sample config if config is absent

Once @mikefab enables TravisCI on this repo, all new pull requests will automatically build against Travis!

(I built this commit on my own fork via Travis already. See the log here.)

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Tagging unicef/magicbox#6 for tracking.