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]>

Gemfile.lock

@@ -76,13 +76,13 @@ GEM
     builder (3.2.3)
     byebug (10.0.2)
     cancancan (2.2.0)
-    capybara (2.18.0)
+    capybara (3.4.2)
       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 (~> 3.1)
     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)
@@ -199,7 +199,7 @@ GEM
     puma (3.12.0)
     rack (2.0.5)
     rack-cors (1.0.2)
-    rack-test (1.0.0)
+    rack-test (1.1.0)
       rack (>= 1.0, < 3)
     rails (5.2.0)
       actioncable (= 5.2.0)
@@ -301,7 +301,7 @@ GEM
     websocket-driver (0.7.0)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.3)
-    xpath (3.0.0)
+    xpath (3.1.0)
       nokogiri (~> 1.8)
 
 PLATFORMS