If one had configured a `teamcity_url` like http://foo.bar/teamcity in
the previous implementation the plugin directed it's request i.e. to
http://foo.bar/httpAuth/... instead of http://foo.bar/teamcity/httpAuth/...

`URI.join` only works correctly, if the prefix URL has
  - at least one or more  trailing '/'
  - the appended parts are _not_ prefixed with '/'

The current implementation should work with all sorts of TeamCity base
URLs.
Signed-off-by: Rémy Coutable <remy@rymai.me>