GPG signature download for rvm and ruby update

RVM download was failing with the following:

```
Downloading https://github.com/wayneeseguin/rvm/archive/1.26.0.tar.gz
Downloading https://github.com/wayneeseguin/rvm/releases/download/1.26.0/1.26.0.tar.gz.asc
Warning, RVM 1.26.0 introduces signed releases and automated check of signatures when GPG software found.
Assuming you trust Michal Papis import the mpapis public key (downloading the signatures).

GPG signature verification failed for '/usr/local/rvm/archives/rvm-1.26.0.tgz' - 'https://github.com/wayneeseguin/rvm/releases/download/1.26.0/1.26.0.tar.gz.asc'!
try downloading the signatures:

    sudo gpg --keyserver hkp://keys.gnupg.net --recv-keys D39DC0E3
```