Travis: Switch codecov with coveralls

[tyll]

Codecov does not seem to properly support multiple jobs but coveralls
does. Therefore use codecov for the CentOS 8 integration tests and
coveralls for everything else.

Signed-off-by: Till Maas opensource@till.name

[codecov]

Codecov Report

Merging #683 into master will increase coverage by 22.54%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #683       +/-   ##
===========================================
+ Coverage   66.25%   88.79%   +22.54%     
===========================================
  Files          27       27               
  Lines        2160     2160               
===========================================
+ Hits         1431     1918      +487     
+ Misses        729      242      -487

Impacted Files	Coverage Δ
libnmstate/validator.py	72.14% <0%> (-22.86%)	⬇️
libnmstate/nm/bond.py	89.28% <0%> (-3.58%)	⬇️
libnmstate/nm/ovs.py	81.45% <0%> (+0.8%)	⬆️
libnmstate/netinfo.py	96.72% <0%> (+1.63%)	⬆️
libnmstate/metadata.py	93.56% <0%> (+6.93%)	⬆️
libnmstate/nm/team.py	100% <0%> (+12.5%)	⬆️
libnmstate/netapplier.py	88.09% <0%> (+23.8%)	⬆️
nmstatectl/nmstatectl.py	79.89% <0%> (+25.77%)	⬆️
libnmstate/nm/ipv4.py	100% <0%> (+26.26%)	⬆️
libnmstate/nm/applier.py	97.44% <0%> (+26.8%)	⬆️
... and 9 more

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[EdDev]

Thank you.