Bump github-api dependency to 1.40

Newer versions of github-api change the way you call connectUsingOAuth
to take the full API url. E.g. "https://api.github.com" or
"https://github.enterprise.com/api/v3"
Add an explicit API URI field to GithubSecurityRealm
Merge pull request #11 from bjeanes/master

Working GitHub Enterprise support
Optimise imports.
Avoid NPE for anonymous admin permissions check

This avoids a potential NPE when jenkins.model.Jenkins.getUser(name) ends
up calling hasPermissions for "hudson.model.Hudson.Administer".  I ran
into this on a jenkins instance that had greenballs-plugin installed, and
READ access for anonymous users enabled.

- Verify read permissions before performing anonymous authorization checks
- Make requestURI return null if Stapler.currentRequest is null
- Make currentUriPathEquals return false if requestURI is null
Depend on latest LTS plugin version