bump releasetools python requirement to 2.7

These scripts already use some post-2.4 features, so let's make it
official: Python 2.7 is needed to run them.

Change-Id: I256e9ed99b0b62abe4e22a7b1f811acb7419e88e
diff --git a/tools/releasetools/make_recovery_patch b/tools/releasetools/make_recovery_patch
index 470dfc4..08d1450 100755
--- a/tools/releasetools/make_recovery_patch
+++ b/tools/releasetools/make_recovery_patch
@@ -16,8 +16,8 @@
 
 import sys
 
-if sys.hexversion < 0x02040000:
-  print >> sys.stderr, "Python 2.4 or newer is required."
+if sys.hexversion < 0x02070000:
+  print >> sys.stderr, "Python 2.7 or newer is required."
   sys.exit(1)
 
 import os