fix Scala 2.13 deprecation warning

[amir734jj]

depreciation warning fixed

amir@amir-desktop:~/workspace/scala-bison$ make compile
mkdir -p bin
(cd src; scalac -d ../bin edu/uwm/cs/util/*.scala edu/uwm/cs/scalabison/*.scala)
edu/uwm/cs/scalabison/BisonScanner.scala:181: warning: method without a parameter list overrides method next in trait Iterator defined with a single empty parameter list [quickfixable]
  /*override*/ def next : BisonTokens.YYToken = state.next;