Template parsing allowing mixed escapes

[apparentlymart]

As reported in hashicorp/terraform-provider-template#54, it looks like the template parsing is allowing %${ and $%{ as escape sequences, rather than requiring the two symbols to match as in $${ and %%{.

[apparentlymart]

This is no longer reproducible on latest master, so I'm going to close this under the assumption that we fixed it in some earlier work and the Terraform template provider was just pinned to an older version.