Set up travis-ci

[h4x3rotab]

I've created the travis-ci project for bitcoingoldjs-lib. The first run shows some (lint?) errors. When we fix this, it's good to replace the CI logo in README.md.

https://travis-ci.org/BTCGPU/bitcoinjs-lib/builds/370664546

[Vutov]

#8

[h4x3rotab]

Integration test is disabled temporarily. We can enable it in the future.