Hello,

this patch updates Guile-SSH[1] to version 0.15.0.

In Guile-SSH 0.15.0 I fixed libguile-ssh library version that I forgot
to bump before the 0.14.0. Now the library version is properly bumped to
"0:14:0" according to libtool rules.

Also I removed a hidden build-time dependency on 'whereis' program from
'util-linux' package.

Note that I haven't properly tested GNU Guix with the new release of
Guile-SSH.

As Guix itself uses Guile-SSH and my update may break something, please
check if it works as expected before merging the update and report any
bugs to me.

- Artyom

References:
1: https://github.com/artyom-poptsov/guile-ssh