Fix optional callback arities

[bryanjos]

When compiling xdr in a project after getting it from hex I ran into the following error:

== Compilation error in file lib/xdr/type/base.ex ==
** (CompileError) lib/xdr/type/base.ex:1: unknown callback length/1 given as optional callback
    (stdlib) lists.erl:1263: :lists.foldl/3
    (stdlib) erl_eval.erl:670: :erl_eval.do_apply/6
could not compile dependency :xdr, "mix compile" failed. You can recompile this dependency with "mix deps.compile xdr", update it with "mix deps.update xdr" or clean it with "mix deps.clean xdr"

I changed the arities in the optional_callback attribute to match those of the callbacks

[sunny-g]

Thank you @bryanjos! Do you wanna bump the patch version as well so I can release a new one (not sure that the OSS best practice is here, you're my first contributor 😄)?

[bryanjos]

No problem. I've update the version to 0.1.1

[sunny-g]

Published - thanks again @bryanjos!