Update build-icon in README.md

As `deepseq` now hosted on GitHub, it now has its own Travis job to allow
validating PRs

README.md

-The `deepseq` Package  [![Build Status](https://travis-ci.org/ghc/packages-deepseq.png?branch=master)](https://travis-ci.org/ghc/packages-deepseq)
+The `deepseq` Package  [![Hackage](https://img.shields.io/hackage/v/deepseq.svg)](https://hackage.haskell.org/package/deepseq) [![Build Status](https://travis-ci.org/haskell/deepseq.svg)](https://travis-ci.org/haskell/deepseq)
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 See [`deepseq` on Hackage](http://hackage.haskell.org/package/deepseq) for more information.