Ignore new star import error F405 in flake8 2.6+, change dev doc to install…

doc/dev_start_guide.txt

@@ -126,7 +126,11 @@ To setup VIM:
 
 #. Install flake8 (if not already installed) with::
 
-    pip install flake8
+    pip install "flake8<3"
+
+   .. warning:: Starting version 3.0.0, flake8 changed its dependancies and 
+      moved its Python API to a legacy module, breaking Theano's flake8 tests.
+      We recommend using a version prior to 3.  
 
    .. note:: You can use ``easy_install`` instead of ``pip``, and ``pep8``
       instead of ``flake8`` if you prefer. The important thing is that the

theano/tests/test_flake8.py

@@ -29,7 +29,8 @@ __contact__ = "Saizheng Zhang <saizhenglisa..at..gmail.com>"
 # - "'absolute_import' present"
 # - "'print_function' present"
 #   Redundant error code generated by flake8-future-import module
-ignore = ('E501', 'E123', 'E133', 'FI12', 'FI14', 'FI15', 'FI16', 'FI17', 'FI50', 'FI51', 'FI53')
+ignore = ('E501', 'E123', 'E133', 'FI12', 'FI14', 'FI15', 'FI16', 'FI17',
+          'FI50', 'FI51', 'FI53', 'F405')
 
 whitelist_flake8 = [
     "compat/six.py",  # This is bundled code that will be deleted, don't fix it