Add keys, required by RVM 1.2.6+

RVM 1.2.6+ introduced signature checking (https://github.com/wayneeseguin/rvm/issues/3110)
The 'vagrant up' and 'vagrant provision' commands fail due this.

Adding the line below before install RVM (on mailcatcher installation) solves the issue:
```
gpg --keyserver hkp://keys.gnupg.net --recv-keys D39DC0E3
```