CI: added build badge in README.md

Author: pdonadeo

.travis.yml

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 os:
   - linux
   - osx
-branches:
-  only:
-  - master

README.md

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 Functional Lenses
 =================
 
+[![Build Status](https://travis-ci.org/pdonadeo/ocaml-lens.svg?branch=master)](https://travis-ci.org/pdonadeo/ocaml-lens)
+
 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.