Fix integration_test script for go env

[chenyukang]

For those who upgraded Go but without setting the right go env (just like me).
Run script will get this error:

yukang@mango:~/youki$  ./integration_test.sh
CGO_ENABLED=0 go build -installsuffix cgo -tags "" -ldflags "-X main.gitCommit=59cdde06764be8d761db120664020f0415f36045 -X main.version=0.9.0" -o runtimetest ./cmd/runtimetest
go: cannot find main module, but found Godeps/Godeps.json in /home/yukang/youki/integration_test/src/github.com/opencontainers/runtime-tools
        to create a module there, run:
        go mod init
make: *** [Makefile:18: runtimetest] Error 1

This PR will fix it.