Update assetgraph-builder to version 4.0.0 🚀

Hello :wave:

:rocket::rocket::rocket:

assetgraph-builder just published its new version 4.0.0, which is not covered by your current version range.

If this pull request passes your tests you can publish your software with the latest version of assetgraph-builder – otherwise use this branch to work on adaptions and fixes.

Happy fixing and merging :palm_tree:

The new version differs by 113 commits .

b28f9ae 4.0.0
a7a8d52 Update assetgraph to 2.0.0 and assetgraph-sprite to 1.0.0.
e287a5c buildProduction: Set assetGraph.sourceMaps to false when --sourcemaps is not specified.
8731753 Merge pull request #238 from assetgraph/greenkeeper-express-processimage-4.2.0
e854f4e autoprefixer transform: Don't break if an asset doesn't produce an existing source map.
5fc359d chore(package): update express-processimage to version 4.2.0
b8d320e gzip transform: Don't create a gzipped version of source maps.
7fc804d processImages transform: Pass on the content type of the source image.
8f442b6 Update express-processimage to 4.1.0.
72f173a Hardwire the assetgraph dep at #master
cae35d1 Sync the assetgraph dep and start passing the sourceMaps option to compressJavaScript.
686dc65 Sync the assetgraph-sprite dep with the tip of the master branch.
d3d6e75 Merge branch 'postcss'
9ca2516 Sync the assetgraph dep with the tip of the master branch now that postcss has been merged.
69b738b Added Fusile test based on @munter's test data, now passes.

There are 113 commits in total. See the full diff.

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Munter / grunt-reduce

Update assetgraph-builder to version 4.0.0 🚀 #22