Fix Node.js 8 compatibility

Fixes #481

Author: sindresorhus

package.json

@@ -48,7 +48,7 @@
 		"regexpp": "^3.0.0",
 		"reserved-words": "^0.1.2",
 		"safe-regex": "^2.1.1",
-		"semver": "^7.1.1"
+		"semver": "^6.3.0"
 	},
 	"devDependencies": {
 		"@lubien/fixture-beta-package": "^1.0.0-beta.1",