.travis.yml: where it matters, have build and source nesting levels differ

.travis.yml: where it matters, have build and source nesting levels differ

Where we build out of source, the source directory was _srcdist and
the build directory was _build.  That gives the same nesting level for
both, which doesn't quite exercise all aspects of relative back
references from build to source tree.

Changing the build tree to be in _build/tree will challenge back
references a bit more, and ensure a bit more that we got it right.

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/11186)