fix Encoding::UndefinedConversionError when for config:set

I was unable to config:set UTF-8 character:

```
15:22:34 + crabman:(master)~/code/donjon >bundle exec ./bin/dj config:set msg="(╯°□°）╯︵ ┻━┻"
Please enter the password for your private key (/home/kim/.ssh/donjon_rsa)
> ************
/home/kim/code/donjon/lib/donjon/encrypted_file.rb:62:in `encode': "\xEA" from ASCII-8BIT to UTF-8 (Encoding::UndefinedConversionError)
	from /home/kim/code/donjon/lib/donjon/encrypted_file.rb:62:in `_encrypt_for'
	from /home/kim/code/donjon/lib/donjon/encrypted_file.rb:36:in `block in write'
	from /home/kim/code/donjon/lib/donjon/user.rb:51:in `call'
	from /home/kim/code/donjon/lib/donjon/user.rb:51:in `block in each'
	from /home/kim/code/donjon/lib/donjon/user.rb:47:in `each'
	from /home/kim/code/donjon/lib/donjon/user.rb:47:in `each'
	from /home/kim/code/donjon/lib/donjon/encrypted_file.rb:35:in `write'
	from /home/kim/code/donjon/lib/donjon/database.rb:19:in `[]='
	from /home/kim/code/donjon/lib/donjon/commands/config.rb:22:in `block in config_set'
	from /home/kim/code/donjon/lib/donjon/commands/config.rb:18:in `each'
	from /home/kim/code/donjon/lib/donjon/commands/config.rb:18:in `config_set'
	from /home/kim/code/donjon/lib/donjon/commands/base.rb:18:in `block in decl'
	from /home/kim/.gem/ruby/2.1.0/gems/thor-0.19.1/lib/thor/command.rb:27:in `run'
	from /home/kim/.gem/ruby/2.1.0/gems/thor-0.19.1/lib/thor/invocation.rb:126:in `invoke_command'
	from /home/kim/.gem/ruby/2.1.0/gems/thor-0.19.1/lib/thor.rb:359:in `dispatch'
	from /home/kim/.gem/ruby/2.1.0/gems/thor-0.19.1/lib/thor/base.rb:440:in `start'
	from /home/kim/code/donjon/lib/donjon/commands/base.rb:14:in `start'
	from ./bin/dj:5:in `<main>'
```

**I'm not sure if this influences the quality of the random_bytes.** I can't find any documentation for `OpenSSL:Random.random_bytes` to find another way to fix this.