mysql 5.6.21: pidof is required only for Mac
mysql 5.6.21: pidof is required only for Mac

Closes Homebrew/linuxbrew#185

LinuxbrewTestBot/homebrew-core,rwhogg/homebrew-core,rwhogg/homebrew-core,forevergenin/homebrew-core,maxim-belkin/homebrew-core,Linuxbrew/homebrew-core,Linuxbrew/homebrew-core,maxim-belkin/homebrew-core,forevergenin/homebrew-core,LinuxbrewTestBot/homebrew-core