message: Update Travis-ci status icon (release branch)
diff: [Website](https://www.bitcoinunlimited.info) | [Download](https://www.bitcoinunlimited.info/download) | [Setup](doc/README.md) | [Xthin](doc/bu-xthin.md) | [Xpedited](doc/bu-xpedited-forwarding.md) | [Miner](doc/miner.md)
  -[![Build Status](https://travis-ci.org/BitcoinUnlimited/BitcoinUnlimited.svg?branch=0.12.1bu)](https://travis-ci.org/BitcoinUnlimited/BitcoinUnlimited)
  +[![Build Status](https://travis-ci.org/BitcoinUnlimited/BitcoinUnlimited.svg?branch=release)](https://travis-ci.org/BitcoinUnlimited/BitcoinUnlimited)
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