MAHOUT-1994: clean viennacl jars on 'mvn clean -Pviennacl -Phadoop2'

[andrewpalumbo]

Purpose of PR:

Please give a short description of what this PR is for.

Important ToDos

Please mark each with an "x"

• [x ] A JIRA ticket exists (if not, please create this first)[https://issues.apache.org/jira/browse/ZEPPELIN/]
• [x ] Title of PR is "MAHOUT-XXXX Brief Description of Changes" where XXXX is the JIRA number.
• Created unit tests where appropriate
• Added licenses correct on newly added files
• [x ] Assigned JIRA to self
• Added documentation in scala docs/java docs, and to website
• Successfully built and ran all unit tests, verified that all tests pass locally.

If all of these things aren't complete, but you still feel it is
appropriate to open a PR, please add [WIP] after MAHOUT-XXXX before the
descriptions- e.g. "MAHOUT-XXXX [WIP] Description of Change"

Does this change break earlier versions?

Is this the beginning of a larger project for which a feature branch should be made?

[rawkintrevo]

lgtm- failed on oracle7 no longer being supported by travis, which has been fixed in another pr