Fix install_requires syntax, allow python 2.6

Author: iMichka

setup.py

@@ -17,7 +17,7 @@
       download_url="https://github.com/gccxml/pygccxml/archive/master.zip",
       license="Boost",
       keywords="C++, declaration parser, gccxml",
-      install_requires=["Python >= 2.7"],
+      install_requires=["Python>=2.6"],
       packages=["pygccxml",
                 "pygccxml.declarations",
                 "pygccxml.parser",