Change contract bytecode location to bytecode.hex

The included download script creates a `contract.hex` which
contains the bytecode. This is nice, but Gigahorse already places
the bytecode in `bytecode.hex`. Let's just use the one
placed there by Gigahorse. Otherwise, it's annoying if that specific
downloader script isn't used.

greed/project.py

@@ -14,7 +14,7 @@
 class Project(object):
     def __init__(self, target_dir: str):
         # Load the contract code
-        with open(f"{target_dir}/contract.hex", "rb") as contract_file:
+        with open(f"{target_dir}/bytecode.hex", "rb") as contract_file:
             self.code = contract_file.read()
 
         self.factory = Factory(project=self)