[snappymail] Some build fixes

Not an expert on the snappy build system, but rollup doesn't seem to be
used anywhere, so no need to check for its presence. Also, I had
to replace "node-fs" with "fs" to build a .tar.gz.

tasks/common.js

@@ -1,7 +1,7 @@
 /* RainLoop Webmail (c) RainLoop Team | Licensed under AGPL 3 */
 const gulp = require('gulp');
 const del = require('del');
-const fs = require('node-fs');
+const fs = require('fs');
 
 const { config } = require('./config');