Add hash of every bundle to build.py.

In bedrock we change the less files a lot,
and have a lot of them. In reading the jingo_minify
code it looked to me like a change to any CSS file
would invalidate all CSS, and the same was true
for js. In my local tests though they seemed to all
be invalidated for every commit. So our long Expires
headers for our media don't do us much good because
all JS and CSS is invalidated on every push, and we
push a few times per week.

The concatinated files are already hashed every time
compress_assets is run to check for sameness. Why not
add those hashes for all bundles to the BUNDLE_HASHES
dict in build.py. The helpers already use it.

I wanted to submit some tests with this as well but
compress_assets is thus for untested. I hope to add
some tests for this command in a subsequent PR.