change travis distro to bionic

[biojppm]

No description provided.

[codecov]

Codecov Report

Merging #59 into master will increase coverage by 1.14%.
The diff coverage is 93.86%.

@@            Coverage Diff             @@
##           master      #59      +/-   ##
==========================================
+ Coverage   91.97%   93.12%   +1.14%     
==========================================
  Files          55       58       +3     
  Lines        5186     5541     +355     
==========================================
+ Hits         4770     5160     +390     
+ Misses        416      381      -35     

Impacted Files	Coverage Δ
test/double_quoted.cpp	100.00% <ø> (ø)
test/empty_map.cpp	100.00% <ø> (ø)
test/empty_seq.cpp	100.00% <ø> (ø)
test/map_of_seq.cpp	100.00% <ø> (ø)
test/merge.cpp	95.65% <0.00%> (-4.35%)	⬇️
test/seq_of_map.cpp	100.00% <ø> (ø)
test/single_quoted.cpp	100.00% <ø> (ø)
test/test_case.hpp	83.87% <ø> (ø)
test/test_group.hpp	100.00% <ø> (ø)
src/c4/yml/common.cpp	71.42% <72.72%> (+21.42%)	⬆️
... and 27 more

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