Fix ROI calculation problem

Prior to that all ROI entries with a key > trade_duration where active.
This causes a problem if the ROI is not linearly declining

freqtrade/strategy/interface.py

@@ -319,17 +319,18 @@ class IStrategy(ABC):
     def min_roi_reached(self, trade: Trade, current_profit: float, current_time: datetime) -> bool:
         """
         Based an earlier trade and current price and ROI configuration, decides whether bot should
-        sell
+        sell. Requires current_profit to be in percent!!
         :return True if bot should sell at current rate
         """
 
         # Check if time matches and current rate is above threshold
-        time_diff = (current_time.timestamp() - trade.open_date.timestamp()) / 60
-        for duration, threshold in self.minimal_roi.items():
-            if time_diff <= duration:
-                continue
-            if current_profit > threshold:
-                return True
+        trade_dur = (current_time.timestamp() - trade.open_date.timestamp()) / 60
+
+        # Get highest entry in ROI dict where key >= trade-duration
+        roi_entry = max(list(filter(lambda x: trade_dur >= x, self.minimal_roi.keys())))
+        threshold = self.minimal_roi[roi_entry]
+        if current_profit > threshold:
+            return True
 
         return False