should add retry mechanism when http request fails

[bruceauyeung]

panic: Get https://api.github.com/repos/kubernetes/test-infra/commits?author=StudyNick&per_page=100: net/http: TLS handshake timeout

goroutine 1 [running]:
panic(0x672940, 0xc047f83260)
D:/Go/src/runtime/panic.go:500 +0x1af
/d/GOPATH/INTERNAL/src/github.com/bruceauyeung/github-contrib-stats/githubstat.getStackalyticsCommits(0xc0420e0ea0, 0xc0422d0100, 0xa, 0xc0422d00d0, 0xa, 0xc0
42039f60, 0x9, 0x0, 0x0, 0x0)
d:/GOPATH/INTERNAL/src/github.com/bruceauyeung/github-contrib-stats/githubstat/metrics_commit_request.go:129 +0x598
/d/GOPATH/INTERNAL/src/github.com/bruceauyeung/github-contrib-stats/githubstat.(*PullRequestMetricsRequest).FetchMetrics(0xc042064058, 0xc0420e8780, 0x0)
d:/GOPATH/INTERNAL/src/github.com/bruceauyeung/github-contrib-stats/githubstat/metrics_pull_request.go:486 +0x5f3
main.main()
d:/GOPATH/INTERNAL/src/github.com/bruceauyeung/github-contrib-stats/main.go:50 +0x418
exit status 2