Update version check to work with >= 4.3

Now that 4.4 is out, the check was failing and not properly setting up
the su daemon.

Using POSIX sh parameter substitution allows us to extract the major and
minor version numbers from ro.build.version.release in build.prop and
compare them.

This was tested on a Nexus 4 running 4.4.
Merge pull request #194 from tmzullinger/kitkat-version-check

Update version check to work with >= 4.3
Merge branch 'master' of https://github.com/koush/Superuser; branch 'master' of github.com:CryosProject/Superuser