Make isFastBoot writeable (again)

[jasonmit]

Discussion: #303

There is a need to allow this prop to be stubbed, currently that is not easy and would involve stubbing self.Fastboot which isn't great since it cannot be reset.

/cc @bcardarella

[rwjblue]

not ok 14 PhantomJS 2.1 - Unit | Service | fastboot in the browser: isFastBoot
    ---
        actual: >
            null
        stack: >
            http://localhost:7357/assets/tests.js:196:24
            exports@http://localhost:7357/assets/vendor.js:136:37
            requireModule@http://localhost:7357/assets/vendor.js:28:25
            require@http://localhost:7357/assets/test-support.js:4710:14
            loadModules@http://localhost:7357/assets/test-support.js:4702:21
            load@http://localhost:7357/assets/test-support.js:4732:33
            http://localhost:7357/assets/test-support.js:4615:22
        message: >
            Expected 3 assertions, but 2 were run
        Log: |
    ...

[jasonmit]

@rwjblue whoops, fixed.

[rwjblue]

Thanks @jasonmit!