Update dependency mathjs to v6.6.3

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change
mathjs (source)	dependencies	patch	6.6.2 -> 6.6.3

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Release Notes

josdejong/mathjs

### [`v6.6.3`](https://togithub.com/josdejong/mathjs/blob/master/HISTORY.md#​2020-04-10-version-663) [Compare Source](https://togithub.com/josdejong/mathjs/compare/v6.6.2...v6.6.3) - Fix [#​1813](https://togithub.com/josdejong/mathjs/issues/1813): bug in engineering notation for numbers of function `format`, sometimes resulting in needless trailing zeros. - Fix [#​1808](https://togithub.com/josdejong/mathjs/issues/1808): methods `.toNumber()` and `.toNumeric()` not working on a unitless unit. - Fix [#​1645](https://togithub.com/josdejong/mathjs/issues/1645): not being able to use named operators `mod`, `and`, `not`, `or`, `xor`, `to`, `in` as object keys. Thanks [@​Veeloxfire](https://togithub.com/Veeloxfire). - Fix `eigs` not using `config.epsilon`.

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