[build] (update) Updated Apache HttpComponents to version 4.3.6

Hello,

I propose to upgrade the *Apache HttpComponents* jars to version 4.3.6 since, as suggested by the [release notes](https://www.apache.org/dist/httpcomponents/httpclient/RELEASE_NOTES-4.3.x.txt):

> Users of all HttpClient versions are advised to upgrade.

In particular,  version 4.3.1 (the one we currently use) suffers of several bugs (most of them have already been solved since version 4.3.2; see the above release note).

NOTE: this new version of Apache HttpComponents still belongs to branch 4.3. There's also a new stable branch 4.4 but it is unclear to me if it is fully back-compatible to branch 4.3. So, in doubt, I would stay with branch 4.3.

Can you consider to merge this pull request?

Cheers,

Marco