Add require 'bundler/gem_tasks' to Rakefile

This PR adds gem tasks to `rake` command.

## Before

```console
% rake -T
rake coverage  # Run RSpec with code coverage
rake spec      # Run RSpec code examples
```

## After

```console
% rake -T
rake build            # Build rubocop-performance-0.0.0.gem into the pkg directory
rake clean            # Remove any temporary products
rake clobber          # Remove any generated files
rake coverage         # Run RSpec with code coverage
rake install          # Build and install rubocop-performance-0.0.0.gem into system gems
rake install:local    # Build and install rubocop-performance-0.0.0.gem into system gems without network access
rake release[remote]  # Create tag v0.0.0 and build and push rubocop-performance-0.0.0.gem to rubygems.org
rake spec             # Run RSpec code examples
```

Rakefile

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require 'bundler'
+require 'bundler/gem_tasks'
 
 begin
   Bundler.setup(:default, :development)