Invoke mvn clean in Travis

[asfimport]

Currently we do not invoke mvn clean in the Travis build which may cause undetected issues in our CI. (See PR #809 for details.)

Reporter: Gabor Szadovszky / @gszadovszky

Related issues:

• Run mvn clean in CI (duplicates)

_{Note: This issue was originally created as PARQUET-1902. Please see the migration documentation for further details.}