Only require `simple_po_parser` in rake task that needs it

That way we don't need to install it in production, since it's really
not needed there.

lib/gitlab/i18n/po_linter.rb

-require 'simple_po_parser'
-
 module Gitlab
   module I18n
     class PoLinter

lib/tasks/gettext.rake

 require "gettext_i18n_rails/tasks"
+require 'simple_po_parser'
 
 namespace :gettext do
   # Customize list of translatable files

spec/lib/gitlab/i18n/po_linter_spec.rb

 require 'spec_helper'
+require 'simple_po_parser'
 
 describe Gitlab::I18n::PoLinter do
   let(:linter) { described_class.new(po_path) }