Fixes technomancy/leiningen#1702 - lein self-install/upgrade/downgrade is broken in lein.bat (2.5.0)

This PR is to fix captioned issue.

As mentioned by @nihilismus, call returns 1. But It's better to use "where" to check the existence of the executable, because 'call' may return previous ERRORLEVEL if the called process didn't set the exit code properly.