configure.ac: simplify the API version number

As we never want to break the API it makes no deeper sence to use a
numbered version '1.0' as this is the same as like a simple '1'.
The defined API number is used within the library so a defined version
'1.0' is also used for the namimg inside the library and other tools
reading them from the library.
Let's reduce the API version to '1' and simplify the naming for the
library by this.

configure.ac

@@ -88,7 +88,7 @@ AC_SUBST(LT_LIBCOAP_AGE)
 AC_SUBST(LIBCOAP_SO_VERSION)
 
 # Defining the API Version
-LIBCOAP_API_VERSION=1.0
+LIBCOAP_API_VERSION=1
 AC_SUBST(LIBCOAP_API_VERSION)
 
 # Adding some default warning options for code QS