CI: added build badge in README.md

Author: pdonadeo

README.md

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 Functional Lenses
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+[![Build Status](https://travis-ci.org/pdonadeo/ocaml-lens.svg?branch=master)](https://travis-ci.org/pdonadeo/ocaml-lens)
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 This package provides some basic types and functions for using lenses in OCaml.
 Functional lenses are based on F# implementation in [FSharpX](https://github.com/fsharp/fsharpx). See [src/FSharpx.Extras/Lens.fs](https://github.com/fsharp/fsharpx/blob/master/src/FSharpx.Extras/Lens.fs) for the original implementation.  Written by Alessandro Strada.