activate_this.py fix

- don't crash if there is no PATH environment variable - in that
case just default to '' and we'll work fine.
- rebuilt virtualenv.py with new ACTIVATE_THIS value

virtualenv.py

@@ -2197,16 +2197,16 @@ def convert(s):
 
 ##file activate_this.py
 ACTIVATE_THIS = convert("""
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 """)
 
 MH_MAGIC = 0xfeedface

virtualenv_embedded/activate_this.py

@@ -13,7 +13,7 @@
 import sys
 import os
 
-old_os_path = os.environ['PATH']
+old_os_path = os.environ.get('PATH', '')
 os.environ['PATH'] = os.path.dirname(os.path.abspath(__file__)) + os.pathsep + old_os_path
 base = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 if sys.platform == 'win32':