gemspec: Exclude various YAML configuration files

[olleolleolle]

This PR makes the gemspec exclude 3 configuration files from the repository's root directory.

The problem that this change solves for me is that a Rake-task-based RuboCop
configuration-file finding method can run into these RuboCop rules, when they
are in a searched filesystem path.

(Another issue: The rules for this project currently present warnings when run with RuboCop 0.49.1.)

The warnings:

/Users/olle/opensource/rake/.rubocop.yml: Style/LineLength has the wrong namespace - should be Metrics
/Users/olle/opensource/rake/.rubocop.yml: Style/IndentationWidth has the wrong namespace - should be Layout
/Users/olle/opensource/rake/.rubocop.yml: Style/Tab has the wrong namespace - should be Layout
/Users/olle/opensource/rake/.rubocop.yml: Style/EmptyLines has the wrong namespace - should be Layout
/Users/olle/opensource/rake/.rubocop.yml: Style/TrailingBlankLines has the wrong namespace - should be Layout
/Users/olle/opensource/rake/.rubocop.yml: Style/TrailingWhitespace has the wrong namespace - should be Layout
/Users/olle/opensource/rake/.rubocop.yml: Style/SpaceBeforeBlockBraces has the wrong namespace - should be Layout
/Users/olle/opensource/rake/.rubocop.yml: Style/SpaceInsideBlockBraces has the wrong namespace - should be Layout
/Users/olle/opensource/rake/.rubocop.yml: Style/SpaceInsideHashLiteralBraces has the wrong namespace - should be Layout
/Users/olle/opensource/rake/.rubocop.yml: Style/CaseIndentation has the wrong namespace - should be Layout
Error: obsolete parameter AlignWith (for Lint/EndAlignment) found in /Users/olle/opensource/rake/.rubocop.yml
`AlignWith` has been renamed to `EnforcedStyleAlignWith`

[hsbt]

Thanks to your suggestion and notice rubocop warnings. I will fix it warnings.