Add possibility to set http_proxy in environment for rbenv install

[marcusphi]

In many enterprises you have to access internet over a proxy. This change uses the standard puppet way to set a http and/or https proxy in the environment of the exec resource. We usually set http_proxy as a global var since it's the same everywhere, that's why the code is like this. If not set, behavior is unchanged. I added stdlib to get concat function.

[marcusphi]

OK. It seems adding stdlib as a dependency requires altering the travis build script.
BTW, Modulefile is deprecated. One should use have a metadata.json instead. Incidentally I have created one.

{
  "name": "alup-rbenv",
  "version": "1.2.0",
  "summary": "rbenv Ruby Version Manager module",
  "author": "alup",
  "description": "Puppet module for rbenv Ruby Version Manager",
  "dependencies": [
    {
      "name": "puppetlabs/stdlib",
      "version_requirement": "4.3.2"
    }
  ],
  "types": [
    {
      "parameters": [
        {
          "name": "name",
          "doc": "Gem qualified name within an rbenv repository"
        },
        {
          "name": "gemname",
          "doc": "The Gem name"
        },
        {
          "name": "ruby",
          "doc": "The ruby interpreter version"
        },
        {
          "name": "rbenv",
          "doc": "The rbenv root"
        },
        {
          "name": "user",
          "doc": "The rbenv owner"
        },
        {
          "name": "source",
          "doc": "The gem source"
        }
      ],
      "providers": [
        {
          "name": "default",
          "doc": "Maintains gems inside an RBenv setup"
        }
      ],
      "name": "rbenvgem"
    }
  ],
  "checksums": {},
  "source": "",
  "project_page": "https://github.com/alup/puppet-rbenv",
  "license": "MIT"
}