Only install and use 0.13.0+ launchers

Prior to this change, `~/.sbt/launchers/0.13.0/sbt-launch.jar` could be
pemanently "spoofed" by an other launcher version, if sbt-extras is
first executed for another version than 0.13.0.

Since -sbt-beta (milestone) and -sbt-snapshot options have been recently
removed, I also stongly simplified the helper functions, like
'make_url'.
Merge commit 'refs/pull/58/head'

Conflicts:
	README.md
Routing ++$version through the same place.