add .no-sublime-package to fix error executing script inside package when installed Package Control in Sublime Text 3

The build command doesn't work with Sublime Text 3 because the script rubymotion_build.sh can't be executed since ST3's Package Control now defaults to not unpacking packages, and instead renaming the zip archive as a .sublime-package. 

Some research suggests that the current error when running the build command in Sublime Text 3 can be fixed by having Packge Control expand the package upon installation, and this is signaled through the file '.no-sublime-package'. [More information here](https://github.com/wbond/sublime_package_control/issues/349).

Unfortunately, in order to test this simple change, I'd have to fork and modify the [Package Control channel repo](https://github.com/wbond/package_control_channel) to refer to my fork of SublimeRubyMotionBuilder, then deploy it so the JSON gets served from an HTTP server so that Package Control picks up the the change. Since adding this file will cause no regressions for users of ST2, I'm making the questionable move of submitting the pull request without confirming the fix myself.