Ensure not to use old concurrent-ruby

It seems that some users encounter with a negative values issue of
`Concurrent.available_processor_count`.

#349 (comment)

To avoid the issue, this PR ensure not to use old `concurrent-ruby`.
This PR also removes considering of `NoMethodError` because we can
assume to use `available_processor_count` always.

lib/parallel.rb

@@ -697,9 +697,10 @@ def instrument_start(item, index, options)
     end
 
     def available_processor_count
+      gem 'concurrent-ruby', '>= 1.3.4'
       require 'concurrent-ruby'
       Concurrent.available_processor_count.floor
-    rescue LoadError, NoMethodError
+    rescue LoadError
       require 'etc'
       Etc.nprocessors
     end