chore(package): update flow-bin to version 0.63.1

[kumar303]

Closes #1187

[rpl]

@kumar303 it looks that flow 0.63.1 is smarter and it is now complaining that we were "lying" in the fake test helper :-)

• https://travis-ci.org/mozilla/web-ext/builds/326406542#L1906-L1909

I propose to workaround the issue by using an additional $FLOW_IGNORE, similarly to the one we are already using to stop flow from complaining about the fake function type signature:

• web-ext/tests/unit/helpers.js

  Line 154 in d16f9c0

  // $FLOW_IGNORE: fake can return any kind of object and fake a defined set of methods for testing.

[kumar303]

Maybe this would work? I haven't tried it yet.

export function fake<T>(...): $Shape<T>

I have a feeling it won't work though because things like Object.keys() tend to confuse Flow easily.

I am fine with a $FLOW_IGNORE comment too.

[coveralls]

Coverage remained the same at 100.0% when pulling d4615b7 on greenkeeper/flow-bin-0.63.1 into d16f9c0 on master.