CI: Add pip install --pre entry to Travis

[effigies]

This gives our dependencies a chance to break a Travis build before breaking everything downstream. I would treat failures of --pre builds as non-blocking for most PRs.

[codecov-io]

Codecov Report

Merging #2197 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2197      +/-   ##
==========================================
- Coverage   72.22%   72.21%   -0.01%     
==========================================
  Files        1171     1171              
  Lines       58620    58620              
  Branches     8434     8434              
==========================================
- Hits        42339    42335       -4     
- Misses      14921    14924       +3     
- Partials     1360     1361       +1

Flag	Coverage Δ
#smoketests	72.21% <ø> (-0.01%)	⬇️
#unittests	69.88% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
nipype/interfaces/base.py	82.94% <0%> (-0.37%)	⬇️

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[satra]

we should figure out a way to do this such that it's not a failure when something breaks here. also we should also do this on circle.

[effigies]

Is the concern PRs showing failure when there is none? Or master showing failure?

[satra]

@effigies - both.

[effigies]

I guess causing a minor disruption before it becomes a major one seems like a feature, to me. I worry that if it's too unobtrusive, it will go unobserved, especially since it's going to be a pretty rare event.

I suppose we could do something where we check for failure, shoot an email, and return success so Travis stays green, though the failure mode for that could get pretty annoying to whoever has the unfortunate inbox.

As to Circle, are there any Python dependencies that only get exercised in Circle?

[satra]

@effigies - alright let's go with this for now and revisit when things break.

[satra]

just a quick note that the flag only works when there is a pre-release on pypi.