Comment the intentional fallthrough

[skaji]

Fix #11

Currently Data-MessagePack-Stream cannot be built on ubuntu 18.04 (which has gcc 7.3.0):

❯ gcc --version
gcc (Ubuntu 7.3.0-16ubuntu3) 7.3.0

❯ perl Makefile.PL
...
❯ make
...
/bin/bash ../libtool  --tag=CC   --mode=compile gcc -DHAVE_CONFIG_H -I. -I..    -I../include -O3 -Wall -Wextra -Werror  -g -MT unpack.lo -MD -MP -MF .deps/unpack.Tpo -c -o unpack.lo unpack.c
libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I.. -I../include -O3 -Wall -Wextra -Werror -g -MT unpack.lo -MD -MP -MF .deps/unpack.Tpo -c unpack.c  -fPIC -DPIC -o unpack.o
In file included from unpack.c:268:0:
../include/msgpack/unpack_template.h: In function ‘template_execute’:
../include/msgpack/unpack_template.h:250:17: error: this statement may fall through [-Werror=implicit-fallthrough=]
                 ++p;
                 ^~~
../include/msgpack/unpack_template.h:252:13: note: here
             default:
             ^~~~~~~
cc1: all warnings being treated as errors

This PR fixes this problem by the same change of msgpack/msgpack-c#586

[typester]

Looks good to me. I didn't know that recent gcc had such a funny warnings though

[typester]

I think we should review #12 too. If @syohex has no time for that, I will do.