Turned off executable bit on test_taxon_filter.py so Travis will run it.

Also removed /usr/bin/env python from test_tools_bwa.py so as not to temp fate.

From "nose" documentation (https://nose.readthedocs.org/en/latest/usage.html): "It is important to note that the default behavior of nose is to not include tests from files which are executable. To include tests from such files, remove their executable bit or use the –exe flag (see ‘Options’ section below)."