Suppress warning by removing unnecessary `lambda` argument

https://github.com/r7kamura/rspec-console/commit/963cf1ea2a8c0dd332b13f2d84c5621be2190a4f is the focus point of this Pull Request.

I changed

```ruby
Object.new.singleton_class.send(:define_method, :foo, lambda) { "bar" }
#=> warning: tried to create Proc object without a block
```

to

```ruby
Object.new.singleton_class.send(:define_method, :foo) { "bar" }
```