Bump capybara from 2.18.0 to 3.15.1

Bumps [capybara](https://github.com/teamcapybara/capybara) from 2.18.0 to 3.15.1.
- [Release notes](https://github.com/teamcapybara/capybara/releases)
- [Changelog](https://github.com/teamcapybara/capybara/blob/master/History.md)
- [Commits](https://github.com/teamcapybara/capybara/compare/2.18.0...3.15.1)

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

Gemfile.lock

@@ -132,13 +132,14 @@ GEM
     builder (3.2.4)
     byebug (11.0.1)
     cancan (1.6.10)
-    capybara (2.18.0)
+    capybara (3.15.1)
       addressable
       mini_mime (>= 0.1.3)
-      nokogiri (>= 1.3.3)
-      rack (>= 1.0.0)
-      rack-test (>= 0.5.4)
-      xpath (>= 2.0, < 4.0)
+      nokogiri (~> 1.8)
+      rack (>= 1.6.0)
+      rack-test (>= 0.6.3)
+      regexp_parser (~> 1.2)
+      xpath (~> 3.2)
     childprocess (3.0.0)
     chronic (0.10.2)
     chunky_png (1.3.14)
@@ -551,6 +552,7 @@ GEM
       nokogiri (~> 1.5)
       optimist (~> 3.0)
     redcarpet (3.5.0)
+    regexp_parser (1.8.2)
     request_store (1.5.0)
       rack (>= 1.4)
     responders (2.4.1)