Update roary_plots from .ix to .loc

Since 0.20.1 .loc is the preferred index lookup method instead of .ix
https://pandas.pydata.org/pandas-docs/version/0.20/whatsnew.html#deprecate-ix

Author: EvdH0

contrib/roary_plots/roary_plots.py

@@ -94,7 +94,7 @@ def get_options():
 
     # Sort the matrix by the sum of strains presence
     idx = roary.sum(axis=1).sort_values(ascending=False).index
-    roary_sorted = roary.ix[idx]
+    roary_sorted = roary.loc[idx]
 
     # Pangenome frequency plot
     plt.figure(figsize=(7, 5))