Bump poltergeist from 1.17.0 to 1.18.1

Bumps [poltergeist](https://github.com/teampoltergeist/poltergeist) from 1.17.0 to 1.18.1.
- [Release notes](https://github.com/teampoltergeist/poltergeist/releases)
- [Changelog](https://github.com/teampoltergeist/poltergeist/blob/master/CHANGELOG.md)
- [Commits](teampoltergeist/poltergeist@v1.17.0...v1.18.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot-support

Gemfile.lock

@@ -76,13 +76,13 @@ GEM
     builder (3.2.3)
     byebug (10.0.2)
     cancancan (2.2.0)
-    capybara (2.18.0)
+    capybara (3.1.0)
       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)
+      xpath (~> 3.0)
     case_transform (0.2)
       activesupport
     climate_control (0.2.0)
@@ -186,8 +186,8 @@ GEM
       terrapin (~> 0.6.0)
     pg (0.21.0)
     phantomjs (2.1.1.0)
-    poltergeist (1.17.0)
-      capybara (~> 2.1)
+    poltergeist (1.18.1)
+      capybara (>= 2.1, < 4)
       cliver (~> 0.3.1)
       websocket-driver (>= 0.2.0)
     pry (0.11.3)