Bump required Python version to 2.6 instead of 2.4

BUGS.txt

@@ -3,9 +3,6 @@ Bugs and Known Problems
 
 AsciiDoc
 --------
-- A benign warning 'with will become a reserved keyword
-  in Python 2.6' sometimes occurs when using Python 2.5 -- it's
-  harmless and will disappear with Python 3.
 - Reported line numbers in diagnostic messages are sometimes wrong.
 - Attribute references in macro attribute lists can't be unescaped
   (with the exception of attribute list entry `{0}`).

INSTALL.txt

@@ -1,7 +1,7 @@
 AsciiDoc Installation
 =====================
 
-NOTE: The current version of AsciiDoc requires *Python 2.4 or newer*
+NOTE: The current version of AsciiDoc requires *Python 2.6 or 2.7*
 to run.  If you don't already have an up-to-date version of Python
 installed it can be downloaded from the official Python website
 http://www.python.org/.

README.asciidoc

@@ -14,7 +14,7 @@ SGML/XML markup) can be customized and extended by the user.
 Prerequisites
 -------------
 AsciiDoc is written in Python so you need a Python interpreter
-(version 2.4 or later) to execute asciidoc(1). Python is installed by
+(version 2.6 or 2.7) to execute asciidoc(1). Python is installed by
 default in most Linux distributions.  You can download Python from the
 official Python website http://www.python.org.
 

asciidoc.py

@@ -11,7 +11,7 @@
 ### Used by asciidocapi.py ###
 VERSION = '8.6.9'           # See CHANGLOG file for version history.
 
-MIN_PYTHON_VERSION = '2.4'  # Require this version of Python or better.
+MIN_PYTHON_VERSION = '2.6'  # Require this version of Python or better.
 
 #---------------------------------------------------------------------------
 # Program constants.