NoMethodError: undefined method `[]' for nil:NilClass

[doriath]

Check full details on Airbrake: #69946661

Backtrace:

```
[PROJECT_ROOT]/lib/online_judges/spoj/problem_page.rb:77:in last_page
[PROJECT_ROOT]/lib/online_judges/spoj/problem_page.rb:19:in problem
[PROJECT_ROOT]/lib/online_judges/english_spoj.rb:25:in fetch_problem
[PROJECT_ROOT]/app/models/problem.rb:31:in find_or_fetch_by
[PROJECT_ROOT]/app/models/online_judge.rb:56:in update_attempt
[PROJECT_ROOT]/app/models/online_judge.rb:17:in block in refresh
[PROJECT_ROOT]/app/models/online_judge.rb:17:in each
[PROJECT_ROOT]/app/models/online_judge.rb:17:in refresh
[PROJECT_ROOT]/lib/tasks/ranking.rake:8:in block (4 levels) in <top (required)>
[GEM_ROOT]/gems/mongoid-3.0.10/lib/mongoid/relations/proxy.rb:117:in each
[GEM_ROOT]/gems/mongoid-3.0.10/lib/mongoid/relations/proxy.rb:117:in method_missing
[GEM_ROOT]/gems/mongoid-3.0.10/lib/mongoid/relations/embedded/many.rb:396:in method_missing
[PROJECT_ROOT]/lib/tasks/ranking.rake:7:in block (3 levels) in <top (required)>
[GEM_ROOT]/gems/mongoid-3.0.10/lib/mongoid/contextual/mongo.rb:645:in yield_document
[GEM_ROOT]/gems/mongoid-3.0.10/lib/mongoid/contextual/mongo.rb:134:in block (2 levels) in each
[GEM_ROOT]/gems/moped-1.2.7/lib/moped/query.rb:78:in block in each
[GEM_ROOT]/gems/moped-1.2.7/lib/moped/cursor.rb:30:in block in each
[GEM_ROOT]/gems/moped-1.2.7/lib/moped/cursor.rb:30:in each
[GEM_ROOT]/gems/moped-1.2.7/lib/moped/cursor.rb:30:in each
[GEM_ROOT]/gems/moped-1.2.7/lib/moped/query.rb:77:in each
[GEM_ROOT]/gems/moped-1.2.7/lib/moped/query.rb:77:in each
[GEM_ROOT]/gems/mongoid-3.0.10/lib/mongoid/contextual/mongo.rb:133:in block in each
[GEM_ROOT]/gems/mongoid-3.0.10/lib/mongoid/contextual/mongo.rb:604:in selecting
[GEM_ROOT]/gems/mongoid-3.0.10/lib/mongoid/contextual/mongo.rb:132:in each
[GEM_ROOT]/gems/mongoid-3.0.10/lib/mongoid/contextual.rb:18:in each
[PROJECT_ROOT]/lib/tasks/ranking.rake:6:in block (2 levels) in <top (required)>
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/task.rb:228:in call
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/task.rb:228:in block in execute
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/task.rb:223:in each
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/task.rb:223:in execute
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/task.rb:166:in block in invoke_with_call_chain
/usr/lib/ruby/1.9.1/monitor.rb:211:in mon_synchronize
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/task.rb:159:in invoke_with_call_chain
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/task.rb:152:in invoke
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/application.rb:143:in invoke_task
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/application.rb:101:in block (2 levels) in top_level
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/application.rb:101:in each
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/application.rb:101:in block in top_level
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/application.rb:110:in run_with_threads
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/application.rb:95:in top_level
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/application.rb:73:in block in run
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/application.rb:160:in standard_exception_handling
[GEM_ROOT]/gems/rake-10.0.3/lib/rake/application.rb:70:in run
[GEM_ROOT]/gems/rake-10.0.3/bin/rake:33:in <top (required)>
[GEM_ROOT]/bin/rake:23:in load
[GEM_ROOT]/bin/rake:23:in <main>
```

[doriath]

Hey @kareth, could you take a look at that?