Bump execjs from 2.8.1 to 2.9.1

Bumps [execjs](https://github.com/rails/execjs) from 2.8.1 to 2.9.1.
- [Release notes](https://github.com/rails/execjs/releases)
- [Commits](https://github.com/rails/execjs/compare/v2.8.1...v2.9.1)

---
updated-dependencies:
- dependency-name: execjs
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

Gemfile

@@ -16,7 +16,7 @@ gem 'rack-protection', '~> 3.0.5'
 gem 'em-websocket', '~> 0.5.3' # WebSocket support
 gem 'uglifier', '~> 4.2'
 gem 'mime-types', '~> 3.4', '>= 3.4.1'
-gem 'execjs', '~> 2.8', '>= 2.8.1'
+gem 'execjs', '~> 2.9'
 gem 'ansi', '~> 1.5'
 gem 'term-ansicolor', :require => 'term/ansicolor'
 gem 'rubyzip', '~> 2.3'

Gemfile.lock

@@ -52,7 +52,7 @@ GEM
     espeak-ruby (1.1.0)
     event_emitter (0.2.6)
     eventmachine (1.2.7)
-    execjs (2.8.1)
+    execjs (2.9.1)
     fiber-local (1.0.0)
     geckodriver-helper (0.24.0)
       archive-zip (~> 0.7)
@@ -241,7 +241,7 @@ DEPENDENCIES
   erubis (~> 2.7)
   espeak-ruby (~> 1.1.0)
   eventmachine (~> 1.2, >= 1.2.7)
-  execjs (~> 2.8, >= 2.8.1)
+  execjs (~> 2.9)
   geckodriver-helper (~> 0.24.0)
   irb (~> 1.8)
   json