Enable flake8

tests/travis/run_test.sh

@@ -52,6 +52,9 @@ if [ ${TASK} == "python_lightweight_test" ]; then
     conda install numpy scipy nose
     python -m pip install graphviz
     python -m nose tests/python/test_basic*.py || exit -1
+    python -m pip install flake8
+    flake8 --ignore E501 python-package || exit -1
+    flake8 --ignore E501 tests/python || exit -1
     exit 0
 fi