fix: Gemfile.lock & Gemfile to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-RUBY-NOKOGIRI-72433

Gemfile

@@ -20,7 +20,7 @@ gem 'tzinfo-data'
 
 group :development do
   gem 'rspec'
-  gem 'poltergeist'
+  gem 'poltergeist', '>= 1.16.0'
   gem 'factory_girl'
   gem 'database_cleaner'
   gem 'launchy'

Gemfile.lock

@@ -25,13 +25,13 @@ GEM
     aws-sigv4 (1.0.2)
     backports (3.8.0)
     bigdecimal (1.3.2)
-    capybara (2.15.1)
+    capybara (3.9.0)
       addressable
       mini_mime (>= 0.1.3)
-      nokogiri (>= 1.3.3)
-      rack (>= 1.0.0)
-      rack-test (>= 0.5.4)
-      xpath (~> 2.0)
+      nokogiri (~> 1.8)
+      rack (>= 1.6.0)
+      rack-test (>= 0.6.3)
+      xpath (~> 3.1)
     cliver (0.3.2)
     coderay (1.1.2)
     concurrent-ruby (1.0.5)
@@ -50,25 +50,25 @@ GEM
       rb-inotify (~> 0.9, >= 0.9.7)
       ruby_dep (~> 1.2)
     method_source (0.9.0)
-    mini_mime (0.1.4)
+    mini_mime (1.0.1)
     mini_portile2 (2.3.0)
     minitest (5.10.3)
     multi_json (1.12.2)
     mustermann (1.0.1)
-    nokogiri (1.8.1)
+    nokogiri (1.8.5)
       mini_portile2 (~> 2.3.0)
-    poltergeist (1.16.0)
-      capybara (~> 2.1)
+    poltergeist (1.18.1)
+      capybara (>= 2.1, < 4)
       cliver (~> 0.3.1)
       websocket-driver (>= 0.2.0)
     pry (0.11.1)
       coderay (~> 1.1.0)
       method_source (~> 0.9.0)
-    public_suffix (3.0.0)
-    rack (2.0.3)
+    public_suffix (3.0.3)
+    rack (2.0.5)
     rack-protection (2.0.0)
       rack
-    rack-test (0.7.0)
+    rack-test (1.1.0)
       rack (>= 1.0, < 3)
     rake (12.1.0)
     rb-fsevent (0.10.2)
@@ -123,9 +123,9 @@ GEM
       addressable
     websocket-driver (0.7.0)
       websocket-extensions (>= 0.1.0)
-    websocket-extensions (0.1.2)
-    xpath (2.1.0)
-      nokogiri (~> 1.3)
+    websocket-extensions (0.1.3)
+    xpath (3.2.0)
+      nokogiri (~> 1.8)
 
 PLATFORMS
   ruby
@@ -138,7 +138,7 @@ DEPENDENCIES
   dotenv
   factory_girl
   launchy
-  poltergeist
+  poltergeist (>= 1.16.0)
   pry
   rake
   rerun
@@ -153,4 +153,4 @@ DEPENDENCIES
   validate_url
 
 BUNDLED WITH
-   1.15.3
+   1.16.6