Proxy not supported in maven artifact retrievals

[rinconjc]

Use the template below when reporting bugs. Please, make sure that
you're running the latest stable clj-refactor.el and that the problem
you're reporting hasn't been reported (and potentially fixed) already.

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Expected behavior

If system proxy is defined, i.e. JVM http.proxyHost, etc, maven search should be done using such proxy

Actual behavior

Maven artefact search, which uses http-kit, ignores JVM proxy settings. It will always use direct connection, and connection errors will be silently ignored. This issued doesn't affect clojars artifact search/listing, because it uses standard java http client (URL).

Steps to reproduce the problem

Run refactor behind a proxy, verify if the org.clojure artifacts are not available in the artifact listing

This is extremely important! Providing us with a reliable way to reproduce
a problem will expedite its solution.

Environment & Version information

Emacs 25, Ubuntu 17.10

clj-refactor.el and refactor-nrepl version information

clj-refactor.el -> 20171117.317
refactor-nrepl -> 2.4.0-SNAPSHOT

You can call cljr-version to obtain this information.

CIDER version information

CIDER 0.17.0snapshot (package: 20180124.1228), nREPL 0.2.13

Include here the version string displayed when
CIDER's REPL is launched. Here's an example:

;; CIDER 0.12.0snapshot (package: 20160331.421), nREPL 0.2.12
;; Clojure 1.8.0, Java 1.8.0_31

Leiningen or Boot version

Leiningen 2.7.1 on Java 1.8.0_151 Java HotSpot(TM) 64-Bit Server VM

Emacs version

25.2.2
E.g. 24.5 (use M-x emacs-version to check it if unsure)

Operating system

Ubuntu 17.10
E.g. Fedora 23, OS X 10.11 "El Capitan", Windows 10, etc