configure.ac: adjust the library vision +1
configure.ac: adjust the library vision +1

We are going into a new release cycle which will provide new so we need
new symbols and interfaces. Because of the new added symbols we need to
increase LT_LIBCOAP_CURRENT to '1'.
And we will be backward compatible and existing binaries linked against
the previous version will also working with that new planned library
release. To show this setting tune LT_LIBCOAP_AGE also to '1' as we are
now one version backward compatible.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap