Bump rubocop-performance from 1.8.0 to 1.8.1

Bumps [rubocop-performance](https://github.com/rubocop-hq/rubocop-performance) from 1.8.0 to 1.8.1.
- [Release notes](https://github.com/rubocop-hq/rubocop-performance/releases)
- [Changelog](https://github.com/rubocop-hq/rubocop-performance/blob/master/CHANGELOG.md)
- [Commits](rubocop/rubocop-performance@v1.8.0...v1.8.1)

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -384,7 +384,7 @@ GEM
     rb-fsevent (0.10.4)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    regexp_parser (1.7.1)
+    regexp_parser (1.8.0)
     request_store (1.5.0)
       rack (>= 1.4)
     rexml (3.2.4)
@@ -399,12 +399,13 @@ GEM
       rubocop-ast (>= 0.4.0, < 1.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 1.4.0, < 2.0)
-    rubocop-ast (0.4.0)
+    rubocop-ast (0.4.2)
       parser (>= 2.7.1.4)
     rubocop-minitest (0.10.1)
       rubocop (>= 0.87)
-    rubocop-performance (1.8.0)
+    rubocop-performance (1.8.1)
       rubocop (>= 0.87.0)
+      rubocop-ast (>= 0.4.0)
     rubocop-rails (2.8.1)
       activesupport (>= 4.2.0)
       rack (>= 1.1)