Update gulp-sourcemaps requirement from ^1.5.2 to ^2.6.4
Update gulp-sourcemaps requirement from ^1.5.2 to ^2.6.4

Updates the requirements on [gulp-sourcemaps](https://github.com/gulp-sourcemaps/gulp-sourcemaps) to permit the latest version.
- [Release notes](https://github.com/gulp-sourcemaps/gulp-sourcemaps/releases)
- [Commits](https://github.com/gulp-sourcemaps/gulp-sourcemaps/commits/v2.6.4)

Signed-off-by: dependabot[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
fengtingzhang/heroku-node-bower,fengtingzhang/heroku-node-bower