Update RoaringBitmap to 0.4.3

Roaring has been updated to version 0.4.3. We fixed a rarely occurring bug with serialization. No API or format changes were made.

Author: Daniel Lemire <lemire@gmail.com>

Closes #2938 from lemire/master and squashes the following commits:

431f3a0 [Daniel Lemire] Recommended bug fix release
[SPARK-3616] Add basic Selenium tests to WebUISuite

This patch adds Selenium tests for Spark's web UI.  To avoid adding extra
dependencies to the test environment, the tests use Selenium's HtmlUnitDriver,
which is pure-Java, instead of, say, ChromeDriver.

I added new tests to try to reproduce a few UI bugs reported on JIRA, namely
SPARK-3021, SPARK-2105, and SPARK-2527.  I wasn't able to reproduce these bugs;
I suspect that the older ones might have been fixed by other patches.

In order to use HtmlUnitDriver, I added an explicit dependency on the
org.apache.httpcomponents version of httpclient in order to prevent jets3t's
older version from taking precedence on the classpath.

I also upgraded ScalaTest to 2.2.1.

Author: Josh Rosen <joshrosen@apache.org>
Author: Josh Rosen <joshrosen@databricks.com>

Closes #2474 from JoshRosen/webui-selenium-tests and squashes the following commits:

fcc9e83 [Josh Rosen] scalautils -> scalactic package rename
510e54a [Josh Rosen] [SPARK-3616] Add basic Selenium tests to WebUISuite.