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

tools/releasetools/check_target_files_signatures

@@ -41,8 +41,8 @@ Usage:  check_target_file_signatures [flags] target_files
 
 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

tools/releasetools/img_from_target_files.py

@@ -31,8 +31,8 @@ Usage:  img_from_target_files [flags] input_target_files output_image_zip
 
 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 errno

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

tools/releasetools/ota_from_target_files

@@ -61,8 +61,8 @@ Usage:  ota_from_target_files [flags] input_target_files output_ota_package
 
 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 copy

tools/releasetools/sign_target_files_apks

@@ -67,8 +67,8 @@ Usage:  sign_target_files_apks [flags] input_target_files output_target_files
 
 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 base64