check if os.getppid is available before using it (#179)

in Windows and Python 2.7, `os.getppid` is not available

closes #179

Author: beniwohli

elasticapm/base.py

@@ -374,7 +374,7 @@ def get_service_info(self):
     def get_process_info(self):
         return {
             'pid': os.getpid(),
-            'ppid': os.getppid(),
+            'ppid': os.getppid() if hasattr(os, 'getppid') else None,
             'argv': sys.argv,
             'title': None,  # Note: if we implement this, the value needs to be wrapped with keyword_field
         }