Previous_occurrence not correctly referencing status

When sensu-server pulls in 'previous_occurrences', it compares the existing status vs. the previous occurrence's one. It appears that this is being improperly referenced. The result is that checks with 'occurrences' set greater than 1 are not getting properly handled.

The status code for the previous occurrence is in previous_occurrence[:check][:status], and not previous_occurrence[:status]. This is easier to explain with a commit: https://github.com/robbydyer/sensu/commit/4aa7c461b530c4467e251b282d52d849f67ffc39