Adds a Metasploit::Concern hook to HttpServer mixin

 I have added a call to `Metasploit::Concern` in the `Msf::Exploit::Remote::HttpServer mixin`, so that the class can be overriden at load time (instead of at some random point before/after modules are loaded).

Note that although I added `metasploit-concern` to the Gemfile, this PR doesn't actually ADD a new dependency, since `Metasploit::Concern` has been pulled in for a long time as a sub-sub-dependency. But since we are referring to it explicitly now, it makes sense to pull into the Gemfile. But bundler shouldn't need to hit the network to pull in anything new. Ideally.

This does not change any behavior in metasploit-framework, but it is worth a quick verification to make sure HttpServer modules still run:

##### Verification
- [x] `bundle install` is fine
- [x] `./msfconsole` starts fine
- [x] run the firefox xpi addon module:

        msf> use exploits/multi/browser/firefox_xpi_bootstrapped_addon
        msf> set LHOST 1.1.1.1
        msf> run -j

- [x] Browse to the URL output from any version of Firefox. You should see a page asking to install an addon.

I'll correct it in the metasploit-framework PR for @jlee-r7's https://github.com/rapid7/metasploit-credential/pull/73