Only convert call target to string if symbol

The only thing this is used for is matching regexes
against known call targets. We only want to store these
strings if the call target is a symbol, not a full Sexp.
Big performance improvement for large apps.

lib/brakeman/call_index.rb

@@ -106,7 +106,7 @@ class Brakeman::CallIndex
   def index_calls calls
     calls.each do |call|
       @methods << call[:method].to_s
-      @targets << call[:target].to_s
+      @targets << call[:target].to_s if call[:target].is_a? Symbol
       @calls_by_method[call[:method]] << call
       @calls_by_target[call[:target]] << call
     end