Fix v0.8.0 compability (issue tomgco#40)

`package.json` in the v0.1.6 release requires the node binary to be of version
`>= 0.5 < 0.7`. This meant that when running `npm install gzippo`, npm would
fall back to v0.0.6, which accepts any node version.

package.json

@@ -14,7 +14,7 @@
     "compress"
   ],
   "engines": {
-    "node": ">= 0.5 < 0.7"
+    "node": ">= 0.5 < 0.9"
   },
   "scripts": {
     "test": "expresso"