Speed up deployment when Ruby is already installed

We were installing RVM every time we were deploying, which meant
deployments took a few extra seconds and generated a lot of unneeded
output even when we didn't have to update Ruby.

We need to update RVM when a new Ruby version needs to be installed
(otherwise we could be asking RVM to install a Ruby version which the
current RVM version cannot handle), but in every other case we're fine
using the already installed RVM version.

We're also considering the case where the RVM command is not available.
Even though the CONSUL installer installs RVM, some people might prefer
to install it through Capistrano.

config/deploy.rb

@@ -39,8 +39,7 @@ namespace :deploy do
   Rake::Task["delayed_job:default"].clear_actions
   Rake::Task["puma:smart_restart"].clear_actions
 
-  after :updating, "rvm1:install:rvm"
-  after :updating, "rvm1:install:ruby"
+  after :updating, "install_ruby"
 
   after "deploy:migrate", "add_new_settings"
 
@@ -60,6 +59,26 @@ namespace :deploy do
   end
 end
 
+task :install_ruby do
+  on roles(:app) do
+    within release_path do
+      begin
+        current_ruby = capture(:rvm, "current")
+      rescue SSHKit::Command::Failed
+        after "install_ruby", "rvm1:install:rvm"
+        after "install_ruby", "rvm1:install:ruby"
+      else
+        if current_ruby.include?("not installed")
+          after "install_ruby", "rvm1:install:rvm"
+          after "install_ruby", "rvm1:install:ruby"
+        else
+          info "Ruby: Using #{current_ruby}"
+        end
+      end
+    end
+  end
+end
+
 task :refresh_sitemap do
   on roles(:app) do
     within release_path do