Fix postinstall script (#538)

Author: MaxGraey

package.json

@@ -53,7 +53,7 @@
     "make": "npm run clean && npm test && npm run build && npm test",
     "all": "npm run check && npm run make",
     "docs": "typedoc --tsconfig tsconfig-docs.json --mode modules --name \"AssemblyScript Compiler API\" --out ./docs/api --ignoreCompilerErrors --excludeNotExported --excludePrivate --excludeExternals --exclude **/std/** --includeDeclarations --readme src/README.md",
-    "postinstall": "opencollective-postinstall"
+    "postinstall": "opencollective-postinstall || exit 0"
   },
   "files": [
     "lib/loader/index.d.ts",