Support the move of the GitHub Teams API #1387

$ go test -v -timeout 30m  ./... -run TestAccGithubTeamMembership_basic
...
2020/01/17 20:22:46 [DEBUG] Github API Request Details:
---[ REQUEST ]---------------------------------------
GET /teams/3606334/memberships/fuzzoli87 HTTP/1.1
Host: api.github.com
User-Agent: go-github
Accept: application/vnd.github.hellcat-preview+json
Accept-Encoding: gzip


-----------------------------------------------------
2020/01/17 20:22:46 [DEBUG] Sleeping 1s between write operations
2020/01/17 20:22:47 [DEBUG] Github API Response Details:
---[ RESPONSE ]--------------------------------------
HTTP/1.1 404 Not Found
Transfer-Encoding: chunked
Access-Control-Allow-Origin: *
Access-Control-Expose-Headers: ETag, Link, Location, Retry-After, X-GitHub-OTP, X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset, X-OAuth-Scopes, X-Accepted-OAuth-Scopes, X-Poll-Interval, X-GitHub-Media-Type
Content-Security-Policy: default-src 'none'
Content-Type: application/json; charset=utf-8
Date: Sat, 18 Jan 2020 01:22:47 GMT
Referrer-Policy: origin-when-cross-origin, strict-origin-when-cross-origin
Server: GitHub.com
Status: 404 Not Found
Strict-Transport-Security: max-age=31536000; includeSubdomains; preload
X-Accepted-Oauth-Scopes: repo
X-Content-Type-Options: nosniff
X-Frame-Options: deny
X-Github-Media-Type: github.v3; param=hellcat-preview; format=json
X-Github-Request-Id: FABE:4655:15D7FC:29DF1A:5E225DE6
X-Oauth-Scopes: admin:gpg_key, admin:org, admin:public_key, admin:repo_hook, delete_repo, repo, user
X-Ratelimit-Limit: 5000
X-Ratelimit-Remaining: 4961
X-Ratelimit-Reset: 1579312573
X-Xss-Protection: 1; mode=block

70
{
 "message": "Not Found",
 "documentation_url": "https://developer.github.com/v3/teams/members/#get-team-membership"
}