TST: test_fast.sh should not run network tests
The convention until recently was that all '@network' tests are marked '@slow' as well,
by the nature of things.
Most/all of the counterexample I looked at are from code last touched in recent weeks.
 if tests are marked `@network` and `@slow` they won't run on any of the travis runs except python 2.6

if

A = set of all pandas tests not marked `@slow`, `-A 'not slow'`
B = set of all pandas tests marked `@slow` but not marked `@network`, `-A 'slow and not network'`
A intersect B = the empty set (cannot be both slow and not slow)

thus no runs of things marked both `@slow` and `@network` (2.6 doesn't have any env args so i'll run things, but not really since no full deps so many (if not all) `@network` tests will not be run)

so either the travis config must be changed (maybe better in the long run, at the cost of using more time) or use this for now
 this is why e.g., the test_fama_french wasn't failing even though it should have
 closing in favor of using detox, works as a drop in replacement for `tox_prll.sh`, thanks @y-p 
