Bump poltergeist from 1.16.0 to 1.18.1

Bumps [poltergeist](https://github.com/teampoltergeist/poltergeist) from 1.16.0 to 1.18.1.
- [Release notes](https://github.com/teampoltergeist/poltergeist/releases)
- [Changelog](https://github.com/teampoltergeist/poltergeist/blob/master/CHANGELOG.md)
- [Commits](https://github.com/teampoltergeist/poltergeist/compare/v1.16.0...v1.18.1)

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

Gemfile.lock

@@ -478,7 +478,7 @@ GEM
       treetop (~> 1.4.8)
     method_source (0.9.0)
     mime-types (1.25.1)
-    mini_mime (0.1.4)
+    mini_mime (1.0.1)
     mini_portile2 (2.1.0)
     momentjs-rails (2.5.1)
       railties (>= 3.1)
@@ -520,8 +520,8 @@ GEM
     paypal-sdk-merchant (1.106.1)
       paypal-sdk-core (~> 0.2.3)
     pg (0.13.2)
-    poltergeist (1.16.0)
-      capybara (~> 2.1)
+    poltergeist (1.18.1)
+      capybara (>= 2.1, < 4)
       cliver (~> 0.3.1)
       websocket-driver (>= 0.2.0)
     polyamorous (0.5.0)
@@ -698,7 +698,7 @@ GEM
       hashdiff
     websocket-driver (0.7.0)
       websocket-extensions (>= 0.1.0)
-    websocket-extensions (0.1.2)
+    websocket-extensions (0.1.3)
     whenever (0.9.2)
       activesupport (>= 2.3.4)
       chronic (>= 0.6.3)