devel/arpc: Upgrade to version 0.6.

It turns out that if you don't explicitly release a piece of software
through GitHub, the checksum of the source tarball may be unstable. This
has also affected ARPC. Version 0.5 and lower now have tarballs with
different checksums.

Version 0.6 is a forced release through GitHub to guarantee a stable
source tarball checksum.

Reviewed by: mat
Differential Revision: https://reviews.freebsd.org/D12431