Rework ci

This MR made some changes to the CI.

• The tests now run under continuumio/anaconda (python 2) and continuumio/anaconda3 (python 3).
• Coverage is still generated from python 3.
• The python 2 tests run all files matching test/*tests.py.

There are tests not being run under python 3 which are now being run under python 2. If we no longer want these, we should remove them.

I think it's worth running the examples in this way as it gives us an immediate check if we've broken any with other changes.

@paul-lasky @moritz.huebner @gregory.ashton

examples/injection_examples/marginalized_likelihood.py

@@ -44,7 +44,8 @@ for key in ['a_1', 'a_2', 'tilt_1', 'tilt_2', 'phi_12', 'phi_jl', 'phase', 'iota
 # This is still under development so care should be taken with the marginalised likelihood.
 likelihood = tupak.gw.GravitationalWaveTransient(
     interferometers=IFOs, waveform_generator=waveform_generator, prior=priors,
-    distance_marginalization=True, phase_marginalization=False)
+    distance_marginalization=False, phase_marginalization=True,
+    time_marginalization=False)
 
 # Run sampler
 result = tupak.run_sampler(likelihood=likelihood, priors=priors, sampler='dynesty',