Update selenium webdriver gem

This makes rspec work again with Firefox v47+

Gemfile

@@ -174,7 +174,7 @@ group :test do
   gem 'fuubar'
   gem 'capybara-select2', github: 'goodwill/capybara-select2'
   gem 'capybara-ng', '~> 0.2.2'
-  gem 'selenium-webdriver', '~> 2.52.0'
+  gem 'selenium-webdriver', '~> 2.53.4'
   gem 'poltergeist'
   gem 'timecop', '~> 0.8'
   gem 'webmock', '~> 1.24.2', require: false

Gemfile.lock

@@ -509,9 +509,8 @@ GEM
       sprockets (>= 2.8, < 4.0)
       sprockets-rails (>= 2.0, < 4.0)
       tilt (>= 1.1, < 3)
-    selenium-webdriver (2.52.0)
+    selenium-webdriver (2.53.4)
       childprocess (~> 0.5)
-      multi_json (~> 1.0)
       rubyzip (~> 1.0)
       websocket (~> 1.0)
     shoulda-context (1.2.1)
@@ -572,7 +571,7 @@ GEM
       addressable (>= 2.3.6)
       crack (>= 0.3.2)
       hashdiff
-    websocket (1.2.2)
+    websocket (1.2.3)
     websocket-driver (0.6.3)
       websocket-extensions (>= 0.1.0)
     websocket-extensions (0.1.2)
@@ -682,7 +681,7 @@ DEPENDENCIES
   rubytree (~> 0.9.7)
   sass (~> 3.4.12)
   sass-rails (~> 5.0.3)
-  selenium-webdriver (~> 2.52.0)
+  selenium-webdriver (~> 2.53.4)
   shoulda-context (~> 1.2)
   shoulda-matchers (~> 3.1)
   simplecov