Binary compatibilty has been broken, update the library version.

Binary compatibilty has been broken, update the library version.

commit 798d53ebb4da4e8e3865ed7bd7f31412fe2be3a7 (`Make it possible to override
info() for functions.') overrides the function::info() virtual method. This
breaks binary compatibility (adds a new entry in the middle of the vtable of
the `function' class).

The experimental proof of the above statement: ginsh linked with the previous
release of libginac segfaults if libginac.so is upgraded to the current
version.

Thereore bump the library version.