Use new pg:backups syntax

We attempted to do this in 5e789c3, but Heroku Toolbelt had some missing
functionality with respect to the new `pg:backups` syntax, and our code
retained the `--expire` flag, which is now automatic and no longer
allowed.

This change requires that users have version 3.32.0 of the Heroku
Toolbelt installed, but the old `pgbackups:capture / pgbackups:restore`
syntax and addon are being retired in the first week of May 2015, so we
need to do this to move on in any event (users who don't upgrade won't
be able to use the old parity either).

Close #29.

lib/parity/backup.rb

@@ -45,7 +45,7 @@ def backup_from
     end
 
     def db_backup_url
-      "heroku pgbackups:url --remote #{from}"
+      "heroku pg:backups public-url --remote #{from}"
     end
 
     def development_db

lib/parity/environment.rb

@@ -31,7 +31,7 @@ def run_via_cli
     end
 
     def backup
-      Kernel.system "heroku pgbackups:capture --expire --remote #{environment}"
+      Kernel.system "heroku pg:backups capture --remote #{environment}"
     end
 
     def deploy

spec/backup_spec.rb

@@ -39,20 +39,20 @@ def database_config_path
   end
 
   def curl_piped_to_pg_restore
-    "curl -s `heroku pgbackups:url --remote production` | #{pg_restore}"
+    "curl -s `heroku pg:backups public-url --remote production` | #{pg_restore}"
   end
 
   def pg_restore
     "pg_restore --verbose --clean --no-acl --no-owner -d parity_development"
   end
 
   def heroku_pass_through
-    "heroku pgbackups:restore DATABASE `heroku pgbackups:url "\
+    "heroku pgbackups:restore DATABASE `heroku pg:backups public-url "\
       "--remote production` --remote staging "
   end
 
   def additional_argument_pass_through
-    "heroku pgbackups:restore DATABASE `heroku pgbackups:url "\
+    "heroku pgbackups:restore DATABASE `heroku pg:backups public-url "\
       "--remote production` --remote staging --confirm thisismyapp-staging"
   end
 end

spec/environment_spec.rb

@@ -171,7 +171,7 @@
   end
 
   def heroku_backup
-    "heroku pgbackups:capture --expire --remote production"
+    "heroku pg:backups capture --remote production"
   end
 
   def heroku_console