Broken readme Travis CI link

[SoulPancake]

https://www.travis-ci.com/bookingcom/carbonapi

Needs fixing, if this is still being actively maintained

[adrianmcphee]

Hi @grzkv, If it's helpful then I'm happy to chat about getting this Carbon API product added to OpenUnited.com and helping to connect these lingering issues with contributors to solve them (they do so in order to build a public portfolio of their work etc)