Gemfile.lock: update testing gems.

Library/Homebrew/test/Gemfile.lock

 GEM
   remote: https://rubygems.org/
   specs:
-    ast (2.3.0)
+    ast (2.4.0)
     codecov (0.1.10)
       json
       simplecov
@@ -9,31 +9,31 @@ GEM
     diff-lcs (1.3)
     docile (1.1.5)
     json (2.1.0)
-    parallel (1.12.0)
-    parallel_tests (2.17.0)
+    parallel (1.12.1)
+    parallel_tests (2.21.2)
       parallel
-    parser (2.4.0.2)
-      ast (~> 2.3)
+    parser (2.5.0.2)
+      ast (~> 2.4.0)
     powerpack (0.1.1)
     rainbow (3.0.0)
-    rspec (3.6.0)
-      rspec-core (~> 3.6.0)
-      rspec-expectations (~> 3.6.0)
-      rspec-mocks (~> 3.6.0)
-    rspec-core (3.6.0)
-      rspec-support (~> 3.6.0)
-    rspec-expectations (3.6.0)
+    rspec (3.7.0)
+      rspec-core (~> 3.7.0)
+      rspec-expectations (~> 3.7.0)
+      rspec-mocks (~> 3.7.0)
+    rspec-core (3.7.1)
+      rspec-support (~> 3.7.0)
+    rspec-expectations (3.7.0)
       diff-lcs (>= 1.2.0, < 2.0)
-      rspec-support (~> 3.6.0)
+      rspec-support (~> 3.7.0)
     rspec-its (1.2.0)
       rspec-core (>= 3.0.0)
       rspec-expectations (>= 3.0.0)
-    rspec-mocks (3.6.0)
+    rspec-mocks (3.7.0)
       diff-lcs (>= 1.2.0, < 2.0)
-      rspec-support (~> 3.6.0)
+      rspec-support (~> 3.7.0)
     rspec-retry (0.5.6)
       rspec-core (> 3.3, < 3.8)
-    rspec-support (3.6.0)
+    rspec-support (3.7.1)
     rspec-wait (0.0.9)
       rspec (>= 3, < 4)
     rubocop (0.52.1)