Add get_package_version support of darwin

When I tried to use `itamae local` on OSX, it failed to execute `get_package_version` with specinfra.
So I'm happy if `get_version` of package for darwin is supported.

## example (for review)

```bash
$ ls -l /usr/local/bin/mysql
lrwxr-xr-x  1 k0kubun  admin  32  7  8 00:26 /usr/local/bin/mysql -> ../Cellar/mysql/5.6.19/bin/mysql
$ brew info mysql
mysql: stable 5.6.21
http://dev.mysql.com/doc/refman/5.6/en/
Conflicts with: mariadb, mysql-cluster, mysql-connector-c, percona-server
/usr/local/Cellar/mysql/5.6.15 (9410 files, 349M)
  Poured from bottle
/usr/local/Cellar/mysql/5.6.16 (9424 files, 349M)
  Poured from bottle
/usr/local/Cellar/mysql/5.6.19 (9536 files, 338M) *
  Poured from bottle
From: https://github.com/Homebrew/homebrew/blob/master/Library/Formula/mysql.rb
...
$ basename $(brew info mysql | grep '\*$' | awk '{print $1}')
5.6.19
```