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

This reverts commit 7825ae7845026302ff0d6cb47d8b60ca63f2d43b.

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.