Updating server dependancies. Also adding github.com/jaytaylor/html2text and gopkg.in/gomail.v2 (#5748)

vendor/github.com/prometheus/common/CONTRIBUTING.md

@@ -2,9 +2,9 @@
 
 Prometheus uses GitHub to manage reviews of pull requests.
 
-* If you have a trivial fix or improvement, go ahead and create a pull
-  request, addressing (with `@...`) one or more of the maintainers
-  (see [AUTHORS.md](AUTHORS.md)) in the description of the pull request.
+* If you have a trivial fix or improvement, go ahead and create a pull request,
+  addressing (with `@...`) the maintainer of this repository (see
+  [MAINTAINERS.md](MAINTAINERS.md)) in the description of the pull request.
 
 * If you plan to do something more involved, first discuss your ideas
   on our [mailing list](https://groups.google.com/forum/?fromgroups#!forum/prometheus-developers).