update Add-on SDK to latest stabilization branch release (1.14b4)

Previously we were on an earlier Add-on SDK stabilization branch build (1.14b1). This change updates to the latest stabilization build (1.14b4) in preparation for eventually updating to the next stable release (1.14) once it becomes available.

In my automatic and manual tests, this change seems to work fine. Note that you have to `git submodule update` and then `make prosthesis` after merging this change in order to update the submodule in your working copy and then copy the latest versions of *bootstrap.js* and *install.rdf* from *addon-sdk/app-extension/* to *addon/template/*.

@rpl, does this look good to you?