[OpenSmalltalk/opensmalltalk-vm] dd2960: Build VMs in different stages.

 
  Branch: refs/heads/Cog
  Home:   https://github.com/OpenSmalltalk/opensmalltalk-vm
  Commit: dd29608d83cab8fcf8ee3b994b32443aa86b60a1
      https://github.com/OpenSmalltalk/opensmalltalk-vm/commit/dd29608d83cab8fcf8ee3b994b32443aa86b60a1
  Author: Fabio Niephaus <[hidden email]>
  Date:   2017-12-17 (Sun, 17 Dec 2017)

  Changed paths:
    M .travis.yml

  Log Message:
  -----------
  Build VMs in different stages.

This way, we reduce the load on the build pipeline. If one stage fails, the next stage is not build.
Build stages docs: https://docs.travis-ci.com/user/build-stages

Also, use an alias for Pharo deployment step and add Pharo file server to SSH known hosts (should fix #160)