Fix to rescue ActiveRecord::ConnectionNotEstablished in available_locales

[takahashim]

Fixes #133

[takahashim]

I'm not sure why the test is failing, but the i18n_backend_spec.rb pass in my local development environment, as shown below:

$ bundle exec rspec spec/lib/decidim/term_customizer/i18n_backend_spec.rb

Randomized with seed 21223

Decidim::TermCustomizer::I18nBackend
  #translations
    returns the correct translations list
  #translate
    calls lookup
    with actual translations
      translates the translation keys correctly
    with plural forms
      translates the translation keys correctly
  #initialized?
    when translations are loaded
      returns true
    when translations are not loaded
      returns false
  #available_locales
    when translations are available
      returns an empty result
    when the translation query raises ActiveRecord::ConnectionNotEstablished
      returns an empty result
    when the translation query raises ActiveRecord::StatementInvalid
      returns and empty result
    when no translations are available
      returns an empty result
  #reload!
    resets the translations

Top 10 slowest examples (2.19 seconds, 16.7% of total time):
  Decidim::TermCustomizer::I18nBackend#available_locales when translations are available returns an empty result
    1.24 seconds ./spec/lib/decidim/term_customizer/i18n_backend_spec.rb:65
  Decidim::TermCustomizer::I18nBackend#translate with plural forms translates the translation keys correctly
    0.92439 seconds ./spec/lib/decidim/term_customizer/i18n_backend_spec.rb:175
  Decidim::TermCustomizer::I18nBackend#translations returns the correct translations list
    0.02311 seconds ./spec/lib/decidim/term_customizer/i18n_backend_spec.rb:137
  Decidim::TermCustomizer::I18nBackend#available_locales when the translation query raises ActiveRecord::ConnectionNotEstablished returns an empty result
    0.00293 seconds ./spec/lib/decidim/term_customizer/i18n_backend_spec.rb:81
  Decidim::TermCustomizer::I18nBackend#initialized? when translations are loaded returns true
    0.00176 seconds ./spec/lib/decidim/term_customizer/i18n_backend_spec.rb:106
  Decidim::TermCustomizer::I18nBackend#translate calls lookup
    0.00151 seconds ./spec/lib/decidim/term_customizer/i18n_backend_spec.rb:143
  Decidim::TermCustomizer::I18nBackend#reload! resets the translations
    0.0012 seconds ./spec/lib/decidim/term_customizer/i18n_backend_spec.rb:121
  Decidim::TermCustomizer::I18nBackend#available_locales when the translation query raises ActiveRecord::StatementInvalid returns and empty result
    0.00087 seconds ./spec/lib/decidim/term_customizer/i18n_backend_spec.rb:71
  Decidim::TermCustomizer::I18nBackend#available_locales when no translations are available returns an empty result
    0.0008 seconds ./spec/lib/decidim/term_customizer/i18n_backend_spec.rb:51
  Decidim::TermCustomizer::I18nBackend#translate with actual translations translates the translation keys correctly
    0.00078 seconds ./spec/lib/decidim/term_customizer/i18n_backend_spec.rb:157

Finished in 13.14 seconds (files took 4.47 seconds to load)
11 examples, 0 failures

Randomized with seed 21223