lib: restore python 2.6 compatibilty

Dropped accidentally in backport @
#1910

The change to >=2.7 was declared semver-major in
#1813

Author: rvagg

lib/find-python.js

@@ -18,7 +18,7 @@ PythonFinder.prototype = {
   log: logWithPrefix(log, 'find Python'),
   argsExecutable: ['-c', 'import sys; print(sys.executable);'],
   argsVersion: ['-c', 'import sys; print("%s.%s.%s" % sys.version_info[:3]);'],
-  semverRange: '2.7.x || >=3.5.0',
+  semverRange: '>=2.6.x || >=3.5.0',
 
   // These can be overridden for testing:
   execFile: cp.execFile,