chore: update dependencies (2018-09-03)

Major bump:

| Package                         | From          | To     |
| :------------------------------ | :------------ | :----- |
| gulp-postcss                    | 7.0.0         | 8.0.0  |
| gulp-babel                      | 8.0.0-beta.2  | 8.0.0  |
| @babel/core                     | 7.0.0-beta.54 | 7.0.0  |
| babel-loader                    | 8.0.0-beta.0  | 8.0.2  |
| @babel/preset-env               | 7.0.0-beta.54 | 7.0.0  |
| browserstack-runner             | 0.7.0         | 0.9.0  |

Minor bump:

| Package                         | From          | To     |
| :------------------------------ | :------------ | :----- |
| @commitlint/cli                 | 7.0.0         | 7.1.1  |
| @commitlint/config-conventional | 7.0.1         | 7.1.1  |
| autoprefixer                    | 9.0.1         | 9.1.3  |
| browser-sync                    | 2.24.6        | 2.24.7 |
| gulp-cache-bust                 | 1.3.0         | 1.4.0  |
| gulp-clean-css                  | 3.9.4         | 3.10.0 |
| gulp-prompt                     | 1.0.1         | 1.1.0  |
| gulp-zip                        | 4.1.0         | 4.2.0  |
| inquirer                        | 6.0.0         | 6.2.0  |
| sinon                           | 6.1.4         | 6.1.5  |
| webpack                         | 4.16.2        | 4.17.2 |
| webpack-stream                  | 5.0.0         | 5.1.1  |

Author: ncoden