Fix bug with --auto-gen-config and RegexpLiteral

The `RegexpLiteral` cop stores a running tally of slashes found inside
regular expressions, which it uses to determine whether the
auto-generated config should disable the cop completely, or just specify
the maximum number of slashes to make the cop pass.

Previously this tally was stored in an instance variable. However, this
doesn't work when run against a project with multiple files, because a
new instance of the cop is created for each file. Thus, we need to store
the running tally in a class-level variable.

cc @tamird