Update com/Acrobot/ChestShop/Signs/ChestShopSign.java
Update com/Acrobot/ChestShop/Signs/ChestShopSign.java

Fixes problems with isValid() when used on a sign which has it's price line formatted with lower case letters. Also, uses RegEx to check the price line is valid.
ChestShop-authors/ChestShop-3,ChestShop-authors/ChestShop-3