downgrade redis to 3.3.3

This fixes the following error seen on metamaps.cc:

> *A* `Gem::LoadError` *occurred while* `POST </mappings>` *was processed
> by* `mappings#create`: Specified 'redis' for Action Cable pubsub
> adapter, but the gem is not loaded. Add `gem 'redis'` to your Gemfile
> (and ensure its version is at the minimum required by Action Cable).

Backtrace
----------------

    ```/usr/local/rvm/rubies/ruby-2.3.0/lib/ruby/2.3.0/monitor.rb:214:in
    `mon_synchronize'
    /home/metamaps/metamaps.cc/app/models/mapping.rb:32:in `after_created'
    /home/metamaps/metamaps.cc/app/controllers/mappings_controller.rb:24:in
    `create'```

Gemfile

@@ -24,7 +24,7 @@ gem 'pundit'
 gem 'pundit_extra'
 gem 'rack-attack'
 gem 'rack-cors'
-gem 'redis'
+gem 'redis', '~> 3.3.3'
 gem 'slack-notifier'
 gem 'snorlax'
 gem 'sucker_punch'

Gemfile.lock

@@ -220,7 +220,7 @@ GEM
     rb-fsevent (0.10.2)
     rb-inotify (0.9.10)
       ffi (>= 0.5.0, < 2)
-    redis (4.0.0)
+    redis (3.3.3)
     responders (2.4.0)
       actionpack (>= 4.2.0, < 5.3)
       railties (>= 4.2.0, < 5.3)
@@ -331,7 +331,7 @@ DEPENDENCIES
   rack-attack
   rack-cors
   rails (~> 5.0.0)
-  redis
+  redis (~> 3.3.3)
   rspec-rails
   rubocop
   sass-rails
@@ -348,4 +348,4 @@ RUBY VERSION
    ruby 2.3.0p0
 
 BUNDLED WITH
-   1.14.6
+   1.15.4