Reactivate tests that were being skipped due to the now fixed Python 2.7 Encoding/Decoding Bug #360

Describe the bug
There are test cases being skipped due to the Python 2.7 text encoding/decoding bug on UNIX systems. Now that this bug is fixed with PR #359 on Issue #351 , these tests can be reactivated.

These tests are being skipped with the following command:

@unittest.skipIf(os.name != "nt" and six.PY2, "encoding/decoding issues with linux py2.7, bug 286536")

The tests that are being skipped are:
TestNGramFeaturizer.test_ngramfeaturizer()
TestLightGbmClassifier.test_lightgbmclassifier()
TestNaiveBayesClassifier.test_naivebayesclassifier()

Desktop (please complete the following information):

• OS: MacOS, Ubuntu 16, Ubuntu 14
• Python: 2.7