Prepare capistrano/db migrations to support more setups (#2)

The following parameters have been added to deploy-secrets.yml:

* **use_rvm**: yes/no
* **ruby_version**: Ruby version for rvm. Defaults to 2.3.2
* **repository**: Git repository. Defaults to the oficial repository
* **branch**: Branch to deploy. Defaults to master
* **rails_env**: Defaults to the stage.

Fixed migrations that required admin rights. Now first check if the postgress extensions are available. If so finish without doing nothin.

Added support for passenger.

db/migrate/20151028213830_add_unaccent_extension.rb

@@ -1,5 +1,11 @@
 class AddUnaccentExtension < ActiveRecord::Migration
   def change
-    execute "create extension if not exists unaccent"
+    return if extension_enabled?('unaccent')
+
+    begin
+      enable_extension 'unaccent'
+    rescue StandardError => e
+      raise "Could not create extension unaccent. Please contact with your system administrator: #{e}"
+    end
   end
 end

db/migrate/20151028221647_add_pg_trgm_extension.rb

@@ -1,5 +1,11 @@
 class AddPgTrgmExtension < ActiveRecord::Migration
   def change
-    execute "create extension if not exists pg_trgm"
+    return if extension_enabled?('pg_trgm')
+
+    begin
+      enable_extension 'pg_trgm'
+    rescue StandardError => e
+      raise "Could not create extension pg_trgm. Please contact with your system administrator: #{e}"
+    end
   end
 end