Correctly detect Rails version when using only parts of the framework

Follow up rubocop/rubocop#11289.

This commit fixes the following failed tests.

```console
% bundle exec rake
(snip)

Failures:

  1) RuboCop::Config#target_rails_version when TargetRailsVersion is not set and gems.locked exists uses
     the single digit Rails version in gems.locked
     Failure/Error: expect(configuration.target_rails_version).to eq 4.1

       expected: 4.1
            got: 5.0

       (compared using ==)
     # ./spec/rubocop/config_spec.rb:88:in `block (6 levels) in <top (required)>'

  2) RuboCop::Config#target_rails_version when TargetRailsVersion is not set and gems.locked exists uses
     the multi digit Rails version in gems.locked
     Failure/Error: expect(configuration.target_rails_version).to eq 400.33

       expected: 400.33
            got: 5.0

       (compared using ==)
     # ./spec/rubocop/config_spec.rb:122:in `block (6 levels) in <top (required)>'

  3) RuboCop::Config#target_rails_version when TargetRailsVersion is not set and Gemfile.lock exists uses
     the multi digit Rails version in Gemfile.lock
     Failure/Error: expect(configuration.target_rails_version).to eq 400.33

       expected: 400.33
            got: 5.0

       (compared using ==)
     # ./spec/rubocop/config_spec.rb:122:in `block (6 levels) in <top (required)>'

  4) RuboCop::Config#target_rails_version when TargetRailsVersion is not set and Gemfile.lock exists uses
     the single digit Rails version in Gemfile.lock
     Failure/Error: expect(configuration.target_rails_version).to eq 4.1

       expected: 4.1
            got: 5.0

       (compared using ==)
     # ./spec/rubocop/config_spec.rb:88:in `block (6 levels) in <top (required)>'

Finished in 0.05343 seconds (files took 1.28 seconds to load)
11 examples, 4 failures

Failed examples:

rspec './spec/rubocop/config_spec.rb[1:1:2:3:1]' # RuboCop::Config#target_rails_version when TargetRailsVersion
is not set and gems.locked exists uses the single digit Rails version in gems.locked
rspec './spec/rubocop/config_spec.rb[1:1:2:3:2]' # RuboCop::Config#target_rails_version when TargetRailsVersion
is not set and gems.locked exists uses the multi digit Rails version in gems.locked
rspec './spec/rubocop/config_spec.rb[1:1:2:2:2]' # RuboCop::Config#target_rails_version when TargetRailsVersion
is not set and Gemfile.lock exists uses the multi digit Rails version in Gemfile.lock
rspec './spec/rubocop/config_spec.rb[1:1:2:2:1]' # RuboCop::Config#target_rails_version when TargetRailsVersion
is not set and Gemfile.lock exists uses the single digit Rails version in Gemfile.lock
```

spec/rubocop/config_spec.rb

@@ -74,6 +74,7 @@
                     bundler (>= 1.3.0, < 2.0)
                     railties (= 4.1.0)
                     sprockets-rails (~> 2.0)
+                  railties (4.1.0)
 
                 PLATFORMS
                   ruby
@@ -108,6 +109,7 @@
                     bundler (>= 1.3.0, < 2.0)
                     railties (= 4.1.0)
                     sprockets-rails (~> 2.0)
+                  railties (400.33.22)
 
                 PLATFORMS
                   ruby