Revert "Use npm instal --force to get around NPM cache corruption bug."

This reverts commit 7825ae7.

test_bundler_npm.js (even after an obvious fix is applied to an unrelated
problem) fails a noticeable percentage of the time in the "bundle multiple apps
in parallel" test. It does not appear to fail on the node-0.10 branch, so will
apply this there instead.

tools/meteor_npm.js

@@ -335,17 +335,8 @@ _.extend(exports, {
     // We don't use npm.commands.install since we couldn't
     // figure out how to silence all output (specifically the
     // installed tree which is printed out with `console.log`)
-    //
-    // We use --force, because the NPM cache is broken! See
-    // https://github.com/isaacs/npm/issues/3265 Basically, switching back and
-    // forth between a tarball fork of version X and the real version X can
-    // confuse NPM. But the main reason to use tarball URLs is to get a fork of
-    // the latest version with some fix, so it's easy to trigger this! So
-    // instead, always use --force. (Even with --force, we still WRITE to the
-    // cache, so we can corrupt the cache for other invocations of npm... ah
-    // well.)
     this._execFileSync(path.join(files.get_dev_bundle(), "bin", "npm"),
-                       ["install", "--force", installArg],
+                       ["install", installArg],
                        {cwd: dir});
   },
 
@@ -355,10 +346,9 @@ _.extend(exports, {
 
     this._ensureConnected();
 
-    // `npm install`, which reads npm-shrinkwrap.json.
-    // see above for why --force.
+    // `npm install`, which reads npm-shrinkwrap.json
     this._execFileSync(path.join(files.get_dev_bundle(), "bin", "npm"),
-                       ["install", "--force"],
+                       ["install"],
                        {cwd: dir});
   },