BUG: Revert returns cov to use ddof of 1.

Fix the spreadsheet to apply a factor of COUNT / COUNT - 1
to the COVAR value.

Also, go back to using the C[1][1] index instead of calculating
var independently.

tests/risk/risk-answer-key-checksums

@@ -1,3 +1,4 @@
 3ac0773c4be4e9e5bacd9c6fa0e03e15
 3a5fae958c8bac684f1773fa8dff7810
 19d580890e211a122e9e746f07c80cbc
+70cfe3677a0ff401c801b8628e125d8f

zipline/finance/risk.py

@@ -482,11 +482,11 @@ class RiskMetricsBase(object):
 
         returns_matrix = np.vstack([self.algorithm_returns,
                                     self.benchmark_returns])
-        C = np.cov(returns_matrix, ddof=0)
+        C = np.cov(returns_matrix, ddof=1)
         eigen_values = la.eigvals(C)
         condition_number = max(eigen_values) / min(eigen_values)
         algorithm_covariance = C[0][1]
-        benchmark_variance = np.var(self.benchmark_returns, ddof=1)
+        benchmark_variance = C[1][1]
         beta = algorithm_covariance / benchmark_variance
 
         return (