trappy/utils: Replace deprecated DataFrame.sort with DataFrame.sort_values

Change-Id: I583e7c899074be772394a977e9e6526277dd7647
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
diff --git a/trappy/utils.py b/trappy/utils.py
index 47ef4ab..a06ff1d 100644
--- a/trappy/utils.py
+++ b/trappy/utils.py
@@ -155,7 +155,7 @@
         data['Time'] = data['Time'][1]
         merged_data.append(data)
 
-    df = pd.concat([pr_df, sec_df], keys=['primary', 'secondary']).sort(columns='__line')
+    df = pd.concat([pr_df, sec_df], keys=['primary', 'secondary']).sort_values(by='__line')
     apply_callback(df, df_fn)
     merged_df = pd.DataFrame.from_dict(merged_data)
     merged_df.set_index('Time', inplace=True)