Travis - Use bionic for the base

[yarikoptic]

we were still using 16.04. For that one we do not even have newer dcm2niix
in NeuroDebian: http://neuro.debian.net/pkgs/dcm2niix.html

With older version test_b0dwi_for_fmap passes (on travis) but I see it
failing locally, so want to see what happens on Travis

[codecov]

Codecov Report

Merging #533 (75b4868) into master (c4956b3) will increase coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #533      +/-   ##
==========================================
+ Coverage   77.55%   77.63%   +0.08%     
==========================================
  Files          41       41              
  Lines        3154     3166      +12     
==========================================
+ Hits         2446     2458      +12     
  Misses        708      708              

Impacted Files	Coverage Δ
heudiconv/tests/test_heuristics.py	100.00% <0.00%> (ø)
heudiconv/bids.py	84.61% <0.00%> (+0.30%)	⬆️

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[yarikoptic]

hm, ok -- we have newer dcm2niix

Get:5 http://neuro.debian.net/debian bionic/main amd64 dcm2niix amd64 1:1.0.20211006-1~nd18.04+1 [224 kB]

and test still passes -- so must be something else about my local environment. Let's proceed with the CI upgrade to bionic then as is

[github-actions]

🚀 PR was released in v0.11.0 🚀