maint: bump version 1.13.1a -> 1.13.2a

maint: bump version 1.13.1a -> 1.13.2a

The 1.13.2 bug-fixing release will ship from the 'branch-1.13.2' git
branch, not from the 'maint' one, since the latter contains changes
that are non-trivial and hasn't cooked enough yet.  The 'maint' branch
will give rise to the 1.13.3 release instead, eventually.  Adjust the
version number to match.

* configure.ac (AC_INIT): Bump version number to 1.13.2b.
* m4/amversion.m4: Likewise (auto-updated by "make bootstrap").

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>