BUG: Use empyrical patches for persisting issue #126

The referenced issue was addressed via importing a set of patches
for empyrical. However the same error occurs occasionally when calling
the empyrical functions inside "/catalyst/finance/risk/period.py".

This PR simply applies 2 of the same patches in period.py.
I have only experienced problems with cum_returns and max_drawdown
thus far, but it is likely the other patches may be needed.

catalyst/finance/risk/period.py

@@ -29,13 +29,15 @@ from .risk import check_entry
 from empyrical import (
     alpha_beta_aligned,
     annual_volatility,
-    cum_returns,
     downside_risk,
     information_ratio,
-    max_drawdown,
     sharpe_ratio,
     sortino_ratio
 )
+from catalyst.patches.stats import (
+    max_drawdown,
+    cum_returns,
+)
 
 from catalyst.constants import LOG_LEVEL