Revert "Stop implying underscore from the meteor-base package."

This reverts commit b513abff23.

Per @abernix's comment here:
https://github.com/meteor/meteor/pull/9362#pullrequestreview-76428132

packages/meteor-base/package.js

@@ -18,6 +18,9 @@ Package.onUse(function(api) {
     // removable
     'webapp',
 
+    // Most Meteor core packages depend on Underscore right now
+    'underscore',
+
     // The protocol and client/server libraries that Meteor uses to send data
     'ddp',
     'livedata', // XXX COMPAT WITH PACKAGES BUILT FOR 0.9.0.