travis: Clean build intermediates before testing.
travis: Clean build intermediates before testing.

As there were some Linux and Mac failures that appear in testing that
appear to be related to resource limitations. It makes sense that we try
and clean any remaining build intermediates. This way we will have more
resources when it comes to building and running tests.

funkey/vigra,funkey/vigra,dstoe/vigra,funkey/vigra,dstoe/vigra,dagophil/vigra,dstoe/vigra,funkey/vigra,dagophil/vigra,dstoe/vigra,funkey/vigra,dagophil/vigra,funkey/vigra,dagophil/vigra,dstoe/vigra,dstoe/vigra,dagophil/vigra,dagophil/vigra,dagophil/vigra,dstoe/vigra,funkey/vigra