ra_machine: Make the version/0 callback return a version() type

[Why]
0 is a valid machine version. It is the default one when the callback is
undefined. However, the callback spec states that a `pos_integer()`
should be returned which forbids a version of 0.

[How]
Simply change the callback return value to `version()`. This makes 0 a
valid return value and reuses the existing type already used elsewhere.

src/ra_machine.erl

@@ -251,7 +251,7 @@
 
 -callback snapshot_module() -> module().
 
--callback version() -> pos_integer().
+-callback version() -> version().
 
 -callback which_module(version()) -> module().