fix(deps): update dependency mathjs to v7 (#775)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [mathjs](https://mathjs.org) ([source](https://github.com/josdejong/mathjs)) | dependencies | major | [`^6.0.0` -> `^7.0.0`](https://renovatebot.com/diffs/npm/mathjs/6.6.5/7.0.0) |

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### Release Notes

<details>
<summary>josdejong/mathjs</summary>

### [`v7.0.0`](https://github.com/josdejong/mathjs/blob/master/HISTORY.md#&#8203;2020-05-07-version-700)

[Compare Source](https://github.com/josdejong/mathjs/compare/v6.6.5...v7.0.0)

Breaking changes:

-   Improvements in calculation of the `dot` product of complex values. 
    The first argument is now conjugated. See [#&#8203;1761](https://github.com/josdejong/mathjs/issues/1761). Thanks [@&#8203;m93a](https://github.com/m93a).  
-   Dropped official support for Node.js v8 which has reached end of life.
-   Removed all deprecation warnings introduced in v6. 
    To upgrade smoothly from v5 to v7 or higher, upgrade to v6 first
    and resolve all deprecation warnings.

</details>

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samples/package.json

@@ -14,7 +14,7 @@
   },
   "dependencies": {
     "@google-cloud/vision": "^1.11.0",
-    "mathjs": "^6.0.0",
+    "mathjs": "^7.0.0",
     "natural": "^2.0.0",
     "pureimage": "^0.2.1",
     "redis": "^3.0.0",