spread: stop testing 17.10

[kyrofa]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• If this is a bugfix. Have you checked that there is a bug report open for the issue you are trying to fix on bug reports?
• If this is a new feature. Have you discussed the design on the forum?
• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh unit?

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Ubuntu 17.10 is EOL as of July 19th, 2018. Stop testing it.

[codecov-io]

Codecov Report

Merging #2197 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2197      +/-   ##
==========================================
+ Coverage   91.13%   91.13%   +<.01%     
==========================================
  Files         201      201              
  Lines       12920    12920              
  Branches     1915     1915              
==========================================
+ Hits        11774    11775       +1     
  Misses        777      777              
+ Partials      369      368       -1

Impacted Files	Coverage Δ
snapcraft/internal/elf.py	83.17% <0%> (+0.32%)	⬆️

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[evandandrea]

Should we also clean up the autopkgtests?

[sergiusens]

@evandandrea there are other open PRs for this (like #2188)