Explicitly add libpath to loadpath

This makes it obsolete to check version against "git" in executables. A nice side effect is that running from git no longer needs `-Ilib` or `RUBYLIB=lib` tricks. `bin/sup` just works.

As for #47, we now only have to modify a verison number in a single place. And we can just check version number into git since the version checking is gone.

* [ ] might want to backport this to line-0.13-develop