message: Update changelog with Mono minimum version requirement
diff: * There currently is no completion support for package references in csproj files. ([#1156](https://github.com/OmniSharp/omnisharp-vscode/issues/1156))
  * As an alternative, consider installing the [MSBuild Project Tools](https://marketplace.visualstudio.com/items?itemName=tintoy.msbuild-project-tools) extension by @tintoy.
  +## 1.15.1 (May 11, 1018)
  +
  +* The minimum Mono version required to run OmniSharp on has been increased to 5.8.1.
  +
  ## 1.15.0 (May 10, 2018)
  #### Debugger
  