Allow pre-2.2 Rubies to execute development restore

In #143, we got a report that because `Etc.nprocessors` was not
implemented until Ruby 2.2 that Ruby 2.1 users were unable to execute
`development restore`, which leverages that method to determine how many
parallel processes to use when restoring the backup from the downloaded
dump.

This change implements a check to see if `Etc.nprocessors` is available.
In the event that it's not, we'll default to two parallel processes. We
will plan to sunset this change in July 2018, giving users on Ruby 2.1,
which has been EOLed, six months to move on.

Close #143.

lib/parity/backup.rb

@@ -72,7 +72,7 @@ def download_remote_backup
     def restore_from_local_temp_backup
       Kernel.system(
         "pg_restore tmp/latest.backup --verbose --clean --no-acl --no-owner "\
-          "--dbname #{development_db} --jobs #{Etc.nprocessors} "\
+          "--dbname #{development_db} --jobs #{processor_cores} "\
           "#{additional_args}",
       )
     end
@@ -106,5 +106,17 @@ def development_db
     def database_yaml_file
       IO.read(DATABASE_YML_RELATIVE_PATH)
     end
+
+    def processor_cores
+      if ruby_version_over_2_2?
+        Etc.nprocessors
+      else
+        2
+      end
+    end
+
+    def ruby_version_over_2_2?
+      Etc.respond_to?(:nprocessors)
+    end
   end
 end

spec/parity/backup_spec.rb

@@ -25,6 +25,30 @@
       with(delete_local_temp_backup_command)
   end
 
+  it "restores backups to development with Rubies that do not support Etc.nprocessors" do
+    allow(IO).to receive(:read).and_return(database_fixture)
+    allow(Kernel).to receive(:system)
+    allow(Etc).to receive(:respond_to?).with(:nprocessors).and_return(false)
+
+    Parity::Backup.new(from: "production", to: "development").restore
+
+    expect(Kernel).
+      to have_received(:system).
+      with(make_temp_directory_command)
+    expect(Kernel).
+      to have_received(:system).
+      with(download_remote_database_command)
+    expect(Kernel).
+      to have_received(:system).
+      with(drop_development_database_drop_command)
+    expect(Kernel).
+      to have_received(:system).
+      with(restore_from_local_temp_backup_command(cores: 2))
+    expect(Kernel).
+      to have_received(:system).
+      with(delete_local_temp_backup_command)
+  end
+
   it "restores backups to staging from production" do
     stub_heroku_app_name
     allow(Kernel).to receive(:system)
@@ -101,9 +125,9 @@ def download_remote_database_command
     'curl -o tmp/latest.backup "$(heroku pg:backups:url --remote production)"'
   end
 
-  def restore_from_local_temp_backup_command
+  def restore_from_local_temp_backup_command(cores: number_of_processes)
     "pg_restore tmp/latest.backup --verbose --clean --no-acl --no-owner "\
-      "--dbname #{default_db_name} --jobs #{number_of_processes} "
+      "--dbname #{default_db_name} --jobs #{cores} "
   end
 
   def number_of_processes