Fix source compatibility with Python 3

According to PEP 394, `python` should only be used in the shebang line
for scripts that are source compatible with both Python 2 and Python 3.

In previous versions of Dotbot, on certain systems where `python`
referred to `python3`, running Dotbot would throw an exception due to a
SyntaxError. This can be fixed by making Dotbot source compatible with
both Python 2 and Python 3.

dotbot/messenger/messenger.py

@@ -14,7 +14,7 @@ def set_level(self, level):
 
     def log(self, level, message):
         if (level >= self._level):
-            print '%s%s%s' % (self._color(level), message, self._reset())
+            print('%s%s%s' % (self._color(level), message, self._reset()))
 
     def debug(self, message):
         self.log(Level.DEBUG, message)