Target node >=5 which supports the spread syntax which is used in the package

[levibuzolic]

The spread syntax has been present in the package for ages, but node 4 doesn't support it. This updates the engines in the package.json to reflect that.

[github-actions]

📊 Package size report   No changes

File	Before	After
Total (Includes all files)	7.4 kB	7.4 kB
Tarball size	3.3 kB	3.3 kB

Unchanged files

File	Size
index.js	106 B
LICENSE	1.1 kB
package.json	807 B
README.md	3.1 kB
rules/no-only-tests.js	2.4 kB

_{🤖 This report was automatically generated by pkg-size-action}