Remove Aragon CLI, document system deps

In order to prevent regular issues with the notoriously npm-issue-ridden
Aragon CLI module, this removes it as a direct dependency and requires
it to be installed as a system dependency.

Also adds Truffle as a system dependency to the docs (which was missing
before).

package.json

@@ -42,7 +42,6 @@
   },
   "homepage": "https://github.com/67P/truffle-kredits#readme",
   "devDependencies": {
-    "@aragon/cli": "^5.9.6",
     "@aragon/kits-base": "^1.0.0",
     "@aragon/os": "^4.2.0",
     "async-each-series": "^1.1.0",