Update dependency gulp-sourcemaps to v3

This PR contains the following updates:

Package	Change	Age	Confidence
gulp-sourcemaps	2.6.5 → 3.0.0

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Release Notes

gulp-sourcemaps/gulp-sourcemaps (gulp-sourcemaps)

v3.0.0: Security Fixes

Compare Source

Nothing changed to the overall API.

However the following security fixes lead to the following:

• acorn which caused us to drop support for Node < 6 https://redirect.github.com/gulp-sourcemaps/gulp-sourcemaps/pull/376/files
• css bump #​380

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