Correctly detect non-platform wheels

[lkollar]

We are not checking if there are any ELF files in the wheel when
inspecting the wheel ABI. This can lead to misleading results when
invoking 'show' and an uncaught exception in other modes.

Resolves: #218

[lkollar]

This is ready for review. The s390x build is still failing due to running out of space on the image: https://travis-ci.community/t/no-space-left-on-device-for-system-z/5954/9.

[codecov]

Codecov Report

Merging #224 into master will decrease coverage by 0.29%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #224     +/-   ##
=========================================
- Coverage   89.54%   89.25%   -0.3%     
=========================================
  Files          19       19             
  Lines         976      996     +20     
  Branches      214      215      +1     
=========================================
+ Hits          874      889     +15     
- Misses         63       65      +2     
- Partials       39       42      +3

Impacted Files	Coverage Δ
auditwheel/main_show.py	72.22% <100%> (-4.87%)	⬇️
auditwheel/main_repair.py	76.59% <100%> (+4.5%)	⬆️
auditwheel/wheel_abi.py	98.4% <100%> (+0.08%)	⬆️
auditwheel/main_addtag.py	86.84% <100%> (+1.54%)	⬆️
auditwheel/repair.py	81.17% <0%> (-2.36%)	⬇️

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