Env: Windows 10 Pro
I installed python 2.7.9 and using pip installed robotframework and robotframework-selenium2library and it all worked fine with no errors.
Then I was doing some research and found that unless there is a reason for me to use 2.x versions of Python, I should stick with 3.x versions. Since 3.4 support already exists for selenium2library (read somewhere), so I decided to switch to it.
I uninstalled python 2.7.9  and installed python 3.4 version. When I installed robotframerwork, I am getting the following:

C:\Users\username>pip install robotframework
  Downloading/unpacking RobotFramework
    Running setup.py (path:C:\Users\username\AppData\Local\Temp\pip_build_username\RobotFramework\setup.py) egg_info for package RobotFramework
      no previously-included directories found matching 'src\robot\htmldata\testdata'
  Installing collected packages: RobotFramework
    Running setup.py install for RobotFramework
        File "C:\Python34\Lib\site-packages\robot\running\timeouts\ironpython.py", line 57
          raise self._error[0], self._error[1], self._error[2]
                              ^
      SyntaxError: invalid syntax
        File "C:\Python34\Lib\site-packages\robot\running\timeouts\jython.py", line 56
          raise self._error[0], self._error[1], self._error[2]
                              ^
      SyntaxError: invalid syntax
      no previously-included directories found matching 'src\robot\htmldata\testdata'
      replacing interpreter in robot.bat and rebot.bat.
  Successfully installed RobotFramework
  Cleaning up...

When I did pip list I do see robotframework is installed. 

C:\Users\username>pip list
pip (1.5.4)
robotframework (3.0)
setuptools (2.1)

Should I be concerned and stick to Python 2.7.9?
With python 2.7.9 you can only install robotframework 2.9
With python 3.X you can install robotframework 3.x+ but as Bryan Oakley said, Selenium2Library is not yet supported ;)
python version for robot framework selenium2library (Windows10)