Fix symbol lexing in Scala lexer (#1438)

The Scala lexer currently matches symbols by ensuring that the last
character in the match is not a `'`.

The problem with this approach is that if the symbol occurs within a
larger structure (e.g. parentheses), the first character after the
symbol (e.g. the closing parenthesis) is matched as part of the symbol.

This commit uses a negative lookahead to avoid this problem.

lib/rouge/lexers/scala.rb

@@ -40,7 +40,7 @@ class Scala < RegexLexer
           groups Keyword, Text
           push :class
         end
-        rule %r/'#{idrest}[^']/, Str::Symbol
+        rule %r/'#{idrest}(?!')/, Str::Symbol
         rule %r/[^\S\n]+/, Text
 
         rule %r(//.*), Comment::Single

spec/visual/samples/scala

@@ -64,4 +64,8 @@ class why_would_you_name_a_class_this_way_oh_well_we_need_to_highlight_it(a: Int
   def hex = 0x123
 }
 
-// Comment at EOF
+StorageState.table(StorageState.NewUsers()).format(
+  keyParams('app_id).asInstanceOf[String]
+)
+
+// Comment at EOF