fix Xcode 9 warnings

[vitalyster]

No description provided.

[codecov-io]

Codecov Report

Merging #251 into master will decrease coverage by 5.38%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #251      +/-   ##
==========================================
- Coverage   99.28%   93.89%   -5.39%     
==========================================
  Files           2        2              
  Lines        1540     1540              
  Branches        0      422     +422     
==========================================
- Hits         1529     1446      -83     
+ Misses         11        7       -4     
- Partials        0       87      +87

Impacted Files	Coverage Δ
YYModel/NSObject+YYModel.m	93.28% <100%> (-5.94%)	⬇️
YYModel/YYClassInfo.m	96.92% <0%> (-2.7%)	⬇️

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