Add OSX to CI

[ianfixes]

Summary

This project will be cross-platform as of #39. Update Travis CI to test both OSX and Linux:

os:
  - linux
  - osx

https://docs.travis-ci.com/user/multi-os/

The, update the build status in the README.md to be a table like in manticore-native

Platform | Version                   | Native | Distro    | Version | CI Status |
-------- | -------                   | ------ |  ------   | ------- |:------    |
iOS      | 7.0+                      | Obj C  | CocoaPods | none    | [![Build Status](https://badges.herokuapp.com/travis/paypal/manticore-native?env=BUILD_ITEM=objc&label=build&branch=master)](https://travis-ci.org/paypal/manticore-native)
MacOS    | 10.9+                     | Obj C  | CocoaPods | none    | [![Build Status](https://badges.herokuapp.com/travis/paypal/manticore-native?env=BUILD_ITEM=objc&label=build&branch=master)](https://travis-ci.org/paypal/manticore-native)
Android  | API 19+                   | Java   | JCenter   | none    | [![Build Status](https://badges.herokuapp.com/travis/paypal/manticore-native?env=BUILD_ITEM=java&label=build&branch=master)](https://travis-ci.org/paypal/manticore-native)
Windows  | XP (.Net4)                | C#     | NuGet     | none    | [![Build status](https://ci.appveyor.com/api/projects/status/sm23ige74y836v48?svg=true)](https://ci.appveyor.com/project/ifreecarve/manticore-native/branch/master)
Windows  | 7,8,Vista (v8  )          | C#     | NuGet     | none    | [![Build status](https://ci.appveyor.com/api/projects/status/sm23ige74y836v48?svg=true)](https://ci.appveyor.com/project/ifreecarve/manticore-native/branch/master)
Windows  | 8.1, 8.1 Phone, 10 (jint) | C#     | NuGet     | none    | [CI not running](https://github.com/paypal/manticore-native/issues/14)
_codegen_| 0.12                      | JS     | npm       | none    | [![Build Status](https://badges.herokuapp.com/travis/paypal/manticore-native?env=BUILD_ITEM=node&label=build&branch=master)](https://travis-ci.org/paypal/manticore-native)