Genome Biology has non standard issue layout

[ryanlower]

Scraping of the paper list from each issue is currently failing:

NoMethodError: undefined method `[]' for nil:NilClass
    from /app/vendor/bundle/ruby/2.1.0/bundler/gems/journals-9c461319d499/lib/journals/bmc_issue.rb:18:in `block in parse_paper_urls'
    from /app/vendor/bundle/ruby/2.1.0/bundler/gems/journals-9c461319d499/lib/journals/bmc_issue.rb:17:in `map'
    from /app/vendor/bundle/ruby/2.1.0/bundler/gems/journals-9c461319d499/lib/journals/bmc_issue.rb:17:in `parse_paper_urls'
    from /app/vendor/bundle/ruby/2.1.0/bundler/gems/journals-9c461319d499/lib/journals/bmc_issue.rb:9:in `parse_papers'
    from /app/vendor/bundle/ruby/2.1.0/bundler/gems/journals-9c461319d499/lib/journals/models/mixins/scrape.rb:26:in `block in parse'
    from /app/vendor/bundle/ruby/2.1.0/bundler/gems/journals-9c461319d499/lib/journals/models/mixins/scrape.rb:25:in `each'
    from /app/vendor/bundle/ruby/2.1.0/bundler/gems/journals-9c461319d499/lib/journals/models/mixins/scrape.rb:25:in `parse'
    from /app/vendor/bundle/ruby/2.1.0/bundler/gems/journals-9c461319d499/lib/journals/models/mixins/scrape.rb:10:in `scrape!'
    from /app/app/models/issue.rb:13:in `scrape!'