WIP: buildProduction: Add support for --browsers switch.

Used to control autoprefixer and the splitCssIfIeLimitIsReached transform.

Removes support for --autoprefix (autoprefixer is now used if and only if --browsers is specified)

Currently requires autoprefixer to be available. Keeping an eye on https://github.com/segmentio/myth/issues/94
jshint nits.
Double mocha's timeout to 120 seconds.

Hopefully that will make the large stylesheets tests pass on Travis.
Up the mocha timeout even further.
Up the timeout even further.
Update assetgraph to 1.7.3.
Remove the enfant terrible test that times out on Travis now that we test that case in assetgraph.
Mock out the splitCssIfIeLimitIsReached transform in the test and assert that it's called correctly.
Set the mocha timeout back to normal.
buildProduction: Remove support for --noinlinecssimageswithlegacyfallback now that there's a better way to express it.
Merge pull request #141 from assetgraph/browsers

buildProduction: Add support for --browsers switch.