Follow Groonga 4.0.3

Groonga 4.0.3 has been released, but rroonga can't find it.

```
$ groonga --version
groonga 4.0.3 [linux-gnu,x86_64,utf8,match-escalation-threshold=0,nfkc,mecab,msgpack,zlib,lzo,epoll]

$ gem i -V rroonga
(snip)
checking for groonga version (>= 4.0.1)... no
downloading http://packages.groonga.org/source/groonga/groonga-4.0.2.tar.gz...^CERROR:  Interrupted
```