Fix error occured when the compared date is invalid

For instance, 2018-09-24 is a holiday in Taiwan so there is no currency record on that date. However, an error would occur on 2018-10-01 when the app computes the currency change in last week because it can't find the corresponding value. This commit makes the app search for the last valid date for computation.

app.R

@@ -96,8 +96,8 @@ server <- function(input, output) {
 
   output$trend <- renderValueBox({
     buy <- subset(target.cur(), 交易種類 == "即期賣出")
-    lastday <- buy$掛牌日期[nrow(buy)]
-    comparedday <- lastday - 7
+    lastday <- max(buy$掛牌日期)
+    comparedday <- max(buy$掛牌日期[buy$掛牌日期 <= lastday - 7])
     changevalue <- round((buy$匯率[nrow(buy)] - buy$匯率[buy$掛牌日期 == comparedday])/
       buy$匯率[buy$掛牌日期 == comparedday] * 100, 2)
     valueBox(