utils: delete docker image with unsupported cmake

[mramotowski]

Pmemfile requires cmake 3.3 or greater and Ubuntu 15.10 provides older version.

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This change is 

[krzycz]

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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

[marcinslusarz]

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

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Comments from Reviewable

[codecov-io]

Codecov Report

Merging #297 into master will increase coverage by 0.03%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #297      +/-   ##
==========================================
+ Coverage    79.4%   79.44%   +0.03%     
==========================================
  Files          81       81              
  Lines       12127    12126       -1     
  Branches     1642     1642              
==========================================
+ Hits         9629     9633       +4     
+ Misses       1891     1888       -3     
+ Partials      607      605       -2

Impacted Files	Coverage Δ
src/libpmemfile-posix/ctree.c	51.07% <0%> (-0.27%)	:arrow_down:
src/libpmemfile/preload.c	44.51% <0%> (-0.07%)	:arrow_down:
src/libpmemfile-posix/rename.c	91.49% <0%> (-0.04%)	:arrow_down:
src/libpmemfile-posix/dir.c	82.06% <0%> (+0.04%)	:arrow_up:
src/libpmemfile-posix/file.c	66.48% <0%> (+0.27%)	:arrow_up:
src/libpmemfile-posix/utils.c	73.43% <0%> (+0.42%)	:arrow_up:
src/libpmemfile/libpmemfile-posix-wrappers.h	50.29% <0%> (+1.18%)	:arrow_up:
src/libpmemfile-posix/read.c	94.33% <0%> (+1.25%)	: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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Comments from Reviewable