Various bug fixes (#2825)

* Fatal error if GPU algorithm selected without GPU support compiled

* Resolve type conversion warnings

* Fix gpu unit test failure

* Fix compressed iterator edge case

* Fix python unit test failures due to flake8 update on pip

tests/travis/run_test.sh

@@ -61,7 +61,7 @@ if [ ${TASK} == "python_lightweight_test" ]; then
     conda install numpy scipy nose
     python -m pip install graphviz
     python -m nose tests/python || exit -1
-    python -m pip install flake8
+    python -m pip install flake8==3.4.1
     flake8 --ignore E501 python-package || exit -1
     flake8 --ignore E501 tests/python || exit -1
     exit 0