Also fix one use of `gsub` that would be faster as `delete`.

Use `tr` instead of `gsub` when you are replacing the same number of
characters. Use `delete` instead of `gsub` when you are deleting
characters.

Also fixes the errors caused by enabling the cop. casecmp is more performant than `.downcase` and `==`.

This reports uses of `File.exists?` and `Dir.exists?`, which were both
deprecated in Ruby and will eventually be removed in favor of
`.exist?`. Also fixes all existing uses of the deprecated methods.

This requires no code changes since it doesn't actually change anything
in the codebase, just preventative.

To 60.

Discussed in #14233.

See [their releases](https://github.com/bbatsov/rubocop/releases) for
more info.

Changes:
- Enable DisplayCopNames for lint output.
- Default behavior for `Alias` changed, set to enforce `prefer_alias_method`.
- Enabling Rails cops changed to new syntax.
- Remove StyleGuides and move Descriptions to comments.
- Add missing cops.
- Add TODOs for cops that should be enabled in the future.
- Set TargetRubyVersion to 2.1.

This enables rubocop metrics like CyclomaticComplexity and ABCSize.
Initial threshold values are high, should be probably decreased.

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: NJeroen van Baarsen <jeroenvanbaarsen@gmail.com>

Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>