Try to fix wrong try usage

`Object#try` checks "whether receiver is nil or not". It doesn't rescue
any exceptions raised in method call.

    > User.new(name: "admin", password_digest: "invalid-hash").authenticate("X")
    BCrypt::Errors::InvalidHash: invalid hash
            from /var/lib/gems/2.1.0/gems/bcrypt-3.1.7/lib/bcrypt/password.rb:60:in `initialize'
            ...
Merge pull request #78 from kou/try-to-fix-wrong-try

Try to fix wrong try usage