This clarifies that the `when_modified` block will be used by both
manual and automatic planning even though it's configured without the
auto planning block. This will help clarif issues like #774.
This changeset has two features:
1. We now automatically download the version of terraform specified in
atlantis.yaml configs if we don't already have that version available
locally.
2. Add a new --default-tf-version flag that allows users to set a
default version of Terraform that we will also download if it's not on
disk.
These mean that users don't need to build custom Docker images to just
add terraform versions. It also means that upgrading the version of
terraform that is packaged with the Atlantis Docker image won't cause
issues for existing users because as long as they're running with
--default-tf-version, Atlantis will always use that version.
