Force validation using packer v1.3.1.

.travis.yml

@@ -7,7 +7,8 @@ env:
   - DIST=trusty
   global:
     # 20170821: as of writing there is no 'latest' (hashicorp/packer/issues/5265)
-  - PACKER_CURRENT_VERSION="$(curl -s https://checkpoint-api.hashicorp.com/v1/check/packer | jq -r -M '.current_version')"
+  - # PACKER_CURRENT_VERSION="$(curl -s https://checkpoint-api.hashicorp.com/v1/check/packer | jq -r -M '.current_version')"
+  - export PACKER_CURRENT_VERSION=1.3.1
   - PACKER_URL="https://releases.hashicorp.com/packer/$PACKER_CURRENT_VERSION/packer_${PACKER_CURRENT_VERSION}_linux_amd64.zip"
   - PACKER_SHA256="https://releases.hashicorp.com/packer/$PACKER_CURRENT_VERSION/packer_${PACKER_CURRENT_VERSION}_SHA256SUMS"
   - PACKER_SHA256_SIG="https://releases.hashicorp.com/packer/$PACKER_CURRENT_VERSION/packer_${PACKER_CURRENT_VERSION}_SHA256SUMS.sig"