I believe I'm havng same problem, this is latest version as of yesterday, installed from source on linux (debian jesse)
./platform
panic: Error opening config file=config.json, err=open config.json: no such file or directory
goroutine 1 0xb)
/home/brw314/code/go/src/github.com/mattermost/platform/utils/config.go:139 +0x16a
main.main()
/home/brw314/code/go/src/github.com/brw31415/platform/mattermost.go:46 +0x43
goroutine 17 [syscall, locked to thread]:
runtime.goexit()
/usr/local/go/src/runtime/asm_amd64.s:1696 +0x1