README: Require GNU Guile 2.0

* README: Require GNU Guile 2.0.
* doc/guile-ssh.texi (Installation): Likewise.
* configure.ac: Remove checks related to GNU Guile 1.8.
* Makefile.am, ssh/Makefile.am: Likewise.
* doc/version.texi: Update.
* NEWS: Update.