Travis-CI passes with unit test errors

[Makoto Yui <my...@apache.org>]

I'm preparing v0.5.2 release and then I found that some unit tests are failing.

Then, I found CI passes with unit test errors in
https://travis-ci.org/apache/incubator-hivemall/jobs/421631300
while CI should fail with unit test errors.

I'm not sure why but this TravisCI behavior should be fixed.

Failed tests:
  GeneralRegressorUDTFTest.testIntegerFeature:202->testFeature:161
  GeneralRegressorUDTFTest.testLazyIntegerFeature:209->testFeature:161
  GeneralRegressorUDTFTest.testLazyLongFeature:230->testFeature:161
  GeneralRegressorUDTFTest.testLazyStringFeature:174->testFeature:161
  GeneralRegressorUDTFTest.testLongFeature:223->testFeature:161
  GeneralRegressorUDTFTest.testStringFeature:181->testFeature:161
  GeneralRegressorUDTFTest.testTextFeature:195->testFeature:161
  GeneralRegressorUDTFTest.testWritableIntFeature:216->testFeature:161
  GeneralRegressorUDTFTest.testWritableLongFeature:237->testFeature:161

It's due to recently introduced code (if there is no gradient, does
not perform update) but actually unit tests need to be fixed.
https://github.com/apache/incubator-hivemall/blob/master/core/src/main/java/hivemall/GeneralLearnerBaseUDTF.java#L460
https://github.com/apache/incubator-hivemall/blob/master/core/src/test/java/hivemall/regression/GeneralRegressorUDTFTest.java#L161

I'm not sure this behavior for initial update strategy where `y` is
zero. Needs some research on some other SGD implementation about it
(random weight initialization so on).

-- 
Makoto YUI <myui AT apache.org>
Research Engineer, Treasure Data, Inc.
http://myui.github.io/