Update package.json.

Remove 'assert' package, which we are no longer using.  Use released
0.9.1 version of es6-shim, instead of our fork.  Also update the rest
of our packages to the latest released minor version, and use "~" versions
consistently so that things don't unexpectedly break if a new major release
occurs.

Change-Id: I356ef64d51362e6d12f82fb8367fc4b7d3472f88