Install latest CMake version

[offa]

Download latest CMake instead of using the version shipped by the package repository.

Background: Older GCC images ship unsupported versions (see #175 discussion).

[codecov[bot]]

Codecov Report

Merging #176 (3fbbcda) into mg/string (77b9e9b) will decrease coverage by 0.67%. The diff coverage is n/a.

@@              Coverage Diff              @@
##           mg/string     #176      +/-   ##
=============================================
- Coverage      34.07%   33.39%   -0.68%     
=============================================
  Files             49       52       +3     
  Lines           2257     2237      -20     
  Branches         336      335       -1     
=============================================
- Hits             769      747      -22     
- Misses          1408     1411       +3     
+ Partials          80       79       -1     

Impacted Files	Coverage Δ
src/main/c/seasocks/ResponseBuilder.cpp	13.33% <0.00%> (-7.88%)	:arrow_down:
src/main/c/Connection.cpp	16.13% <0.00%> (-1.38%)	:arrow_down:
src/main/c/Server.cpp	19.66% <0.00%> (-1.16%)	:arrow_down:
src/main/c/HybiPacketDecoder.cpp	72.58% <0.00%> (-0.86%)	:arrow_down:
src/main/c/seasocks/ZlibContext.cpp	3.27% <0.00%> (-0.12%)	:arrow_down:
src/main/c/seasocks/util/Html.h	98.46% <0.00%> (-0.05%)	:arrow_down:
src/main/c/PageRequest.cpp	0.00% <0.00%> (ø)
src/main/c/seasocks/SimpleResponse.h	0.00% <0.00%> (ø)
src/main/c/seasocks/util/RootPageHandler.h	0.00% <0.00%> (ø)
src/main/c/seasocks/ZlibContext.h	100.00% <0.00%> (ø)
... and 1 more

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[offa]

See #174