Fix completion_spec on MRI 2.7.0

The spec was failing because on 2.7 hash directly return nil without
hashing the key if they are empty:

```ruby
m = Module.new do
  def self.hash; end
end

h = {}
h[m] # => nil
h[1] = 2
h[m] # => ArgumentError
```

So we need to require IRB so that the to_ignore set isn't empty

1 files changed, 1 insertions, 1 deletions

diff --git a/spec/completion_spec.rb b/spec/completion_spec.rb
index 5df577f5..86aa41b5 100644
--- a/spec/completion_spec.rb
+++ b/spec/completion_spec.rb
@@ -239,7 +239,7 @@ describe Pry::InputCompleter do
   unless Pry::Helpers::Platform.jruby?
     # Classes that override .hash are still hashable in JRuby, for some reason.
     it 'ignores methods from modules that override Object#hash incompatibly' do
-      # skip unless Pry::Helpers::Platform.jruby?
+      require 'irb'
 
       m = Module.new do
         def self.hash; end