Travis tests: fix for py.test directory cache
Travis tests: fix for py.test directory cache

py.test raises an error when running tests inside and outside of Docker
with the same test tree:
```
ImportMismatchError: ('tests.conftest', '/bcbio/tests/conftest.py',
local('/home/travis/build/chapmanb/bcbio-nextgen/tests/conftest.py'))
```
I'm not sure how/where it caches the external and internal locations but
work around the issue by mounting to same directory inside and outside
of Docker.

chapmanb/bcbio-nextgen,biocyberman/bcbio-nextgen,vladsaveliev/bcbio-nextgen,brainstorm/bcbio-nextgen,a113n/bcbio-nextgen,biocyberman/bcbio-nextgen,brainstorm/bcbio-nextgen,lbeltrame/bcbio-nextgen,brainstorm/bcbio-nextgen,lbeltrame/bcbio-nextgen,lbeltrame/bcbio-nextgen,biocyberman/bcbio-nextgen,chapmanb/bcbio-nextgen,a113n/bcbio-nextgen,chapmanb/bcbio-nextgen,vladsaveliev/bcbio-nextgen,vladsaveliev/bcbio-nextgen,a113n/bcbio-nextgen