travis: don't use deprecated image

[marcinslusarz]

Not needed probably since #303.

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This change is 

[krzycz]

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Review status: 0 of 1 files reviewed at latest revision, all discussions resolved.

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[krzycz]

Reviewed 1 of 1 files at r1. Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[codecov[bot]]

Codecov Report

Merging #349 into master will increase coverage by 0.01%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #349      +/-   ##
=========================================
+ Coverage   74.09%   74.1%   +0.01%     
=========================================
  Files          64      64              
  Lines        7999    7999              
  Branches     1617    1617              
=========================================
+ Hits         5927    5928       +1     
  Misses       1562    1562              
+ Partials      510     509       -1

Flag	Coverage Δ
#sqlite_tests	45.87% <ø> (ø)	:arrow_up:
#tests_antool	12.19% <ø> (ø)	:arrow_up:
#tests_posix_multi_threaded	25.93% <ø> (-0.02%)	:arrow_down:
#tests_posix_single_threaded	55.97% <ø> (-0.03%)	:arrow_down:
#tests_preload	46.14% <ø> (-0.06%)	:arrow_down:

Impacted Files	Coverage Δ
src/libpmemfile-posix/file.c	70.67% <0%> (-0.28%)	:arrow_down:
src/libpmemfile-posix/dir.c	85.74% <0%> (+0.49%)	:arrow_up:

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[sarahjelinek]

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Reviewed 1 of 1 files at r1. Review status: :shipit: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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