Update dependency gulp-babel to v8

[renovate[bot]]

This PR contains the following updates:

Package	Type	Update	Change	References
gulp-babel	devDependencies	major	7.0.1 -> 8.0.0	source

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Release Notes

babel/gulp-babel

### [`v8.0.0`](https://togithub.com/babel/gulp-babel/blob/master/CHANGELOG.md#​800-beta2-2018-03-14) [Compare Source](https://togithub.com/babel/gulp-babel/compare/v7.0.1...v8.0.0) - **[Fix]** Fix for `sourceMapTarget` removal in Babel v7.0.0-beta.41 ([#​149](https://togithub.com/babel/gulp-babel/pull/149))

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