install-protoc was checking version from wrong executable path

[temoto]

Thanks for install-protoc script, I copied it to my project.

[codecov-io]

Codecov Report

Merging #721 into master will decrease coverage by 0.33%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #721      +/-   ##
==========================================
- Coverage   56.16%   55.83%   -0.34%     
==========================================
  Files          30       30              
  Lines        3112     3161      +49     
==========================================
+ Hits         1748     1765      +17     
- Misses       1193     1223      +30     
- Partials      171      173       +2

Impacted Files	Coverage Δ
protoc-gen-grpc-gateway/descriptor/registry.go	68.06% <0%> (-2.98%)	⬇️
runtime/handler.go	41.6% <0%> (-1.74%)	⬇️
protoc-gen-grpc-gateway/descriptor/services.go	73.97% <0%> (-1.57%)	⬇️
protoc-gen-swagger/main.go	27.05% <0%> (-0.33%)	⬇️
runtime/mux.go	44.16% <0%> (ø)	⬆️
protoc-gen-grpc-gateway/descriptor/types.go	49.07% <0%> (ø)	⬆️
protoc-gen-swagger/genswagger/template.go	38.99% <0%> (+0.23%)	⬆️

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