Ruby: Ignore .irb_history file

[olliebennett]

Reasons for making this change:

In a Ruby IRB ("interactive Ruby") console session, my commands are persisted in a file named .irb_history; by default the file stores the last 1,000 commands.

It doesn't make sense to check my IRB history into a repository.

Links to documentation supporting these rule changes:

https://ruby-doc.org/stdlib-2.4.0/libdoc/irb/rdoc/IRB.html#module-IRB-label-History