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

`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 Bamboo base URLs.
Signed-off-by: Rémy Coutable <remy@rymai.me>