Makefile: some cleanup

 * add `test-ember-09` and `test-ember-1` to `PHONY` to prevent
   Make from expecting those targets from creating files of the
   same names
 * extract `PHANTOM_JS` executable path for reuse in the two test
   targets
 * define `$(JSHINT)` and `$(PHANTOM_JS)` targets that depend on
   `npm_install` and use those targets as prerequisites where
   appropriate