Fix links to docs (#1868)

* Fix links to docs

The docs still don't exist yet. This commit will trigger a travis build.

* Corrected link

README.md

@@ -43,7 +43,7 @@ Our documentation includes an installation guide, quick-start guide, and referen
 a list of the significant breaking changes in the JuMP 0.19 release.**
 
 **Latest Release**: 0.19.0 (`release-0.19` branch)
-  * [Documentation](http://www.juliaopt.org/JuMP.jl/0.19/)
+  * [Documentation](http://www.juliaopt.org/JuMP.jl/v0.19.0/)
   * [Examples](https://github.com/JuliaOpt/JuMP.jl/tree/release-0.19/examples)
   * Testing status:
     * TravisCI: [![Build Status](https://travis-ci.org/JuliaOpt/JuMP.jl.svg?branch=release-0.19)](https://travis-ci.org/JuliaOpt/JuMP.jl)