Update Mac Os for Travis CI (#1196), fixes #1195
Update Mac Os for Travis CI (#1196), fixes #1195

More information: 
https://docs.travis-ci.com/user/reference/osx/
Ky7m/BenchmarkDotNet,adamsitnik/BenchmarkDotNet,adamsitnik/BenchmarkDotNet,PerfDotNet/BenchmarkDotNet,adamsitnik/BenchmarkDotNet,PerfDotNet/BenchmarkDotNet,Ky7m/BenchmarkDotNet,Ky7m/BenchmarkDotNet,PerfDotNet/BenchmarkDotNet,Ky7m/BenchmarkDotNet,adamsitnik/BenchmarkDotNet