Remove .npmignore and instead use the files field

The npmignore meant that we were publishing both source and compiled
files, along with ... everything else that wasn't ignored.

While this was probably great for local debugging, it's potentially
insecure and a bad practice. Instead, only publish our dist folder.

Author: rylnd

.npmignore

@@ -17,6 +17,9 @@
   "author": "",
   "license": "MIT",
   "main": "dist/index.js",
+  "files": [
+    "dist"
+  ],
   "dependencies": {
     "express": "^4.16.4",
     "gm": "^1.16.0",