Pin mongodb package to 1.4.28
Pin mongodb package to 1.4.28

pow-mongodb-fixtures doesn't work with newer versions of mongodb because
of a change in the way listCollections works [1] which conflicts with a
workaround that pow-mongodb-fixtures was doing [2].

This change can be reverted if/when pow-mongodb-fixtures is updated to
pin it's version of mongodb to a specific version.

1: https://github.com/mongodb/node-mongodb-native/blob/bfbfdf1a2810f2288484990ed70860b3e04ca5af/HISTORY#L14
2: https://github.com/powmedia/pow-mongodb-fixtures/blob/415c1d8ed7f381228c6a385b065045ae98e14ec8/src/index.js#L158-L159

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