Don't create test failure.

theano/sandbox/cuda/__init__.py

-raise RuntimeError(
+from nose.plugins.skip import SkipTest
+
+raise SkipTest(
     "You are importing theano.sandbox.cuda. This is the old GPU back-end and "
     "is removed from Theano. Use Theano 0.9 to use it. Even better, "
     "transition to the new GPU back-end! See "

theano/tests/test_flake8.py

@@ -73,6 +73,7 @@ whitelist_flake8 = [
     "scalar/__init__.py",
     "scalar/tests/__init__.py",
     "sandbox/__init__.py",
+    "sandbox/cuda/__init__.py",
     "sandbox/tests/__init__.py",
     "sandbox/gpuarray/__init__.py",
     "sandbox/linalg/__init__.py",