renames config to configuration (#198)

This is not the right reason to be making this kind of change, but a very hard to debug symbol clash in codex for `config`. Changing this to `configuration` is the easiest way to fix the issue.

contracts/Marketplace.sol

@@ -88,7 +88,7 @@ contract Marketplace is SlotReservations, Proofs, StateRetrieval, Endian {
     _config = configuration;
   }
 
-  function config() public view returns (MarketplaceConfig memory) {
+  function configuration() public view returns (MarketplaceConfig memory) {
     return _config;
   }