Try using 1.3.3 workaround.

.travis.yml

@@ -7,8 +7,7 @@ env:
   - DIST=trusty
   global:
     # 20170821: as of writing there is no 'latest' (hashicorp/packer/issues/5265)
-  - # PACKER_CURRENT_VERSION="$(curl -s https://checkpoint-api.hashicorp.com/v1/check/packer | jq -r -M '.current_version')"
-  - export PACKER_CURRENT_VERSION=1.3.1
+  - PACKER_CURRENT_VERSION="$(curl -s https://checkpoint-api.hashicorp.com/v1/check/packer | jq -r -M '.current_version')"
   - 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"
@@ -29,7 +28,8 @@ install:
 
 script:
 - ./packer --version
-- ./packer validate  -var "iso_url=https://downloads.archlinux.de/iso/$(date +'%Y.%m').01/archlinux-$(date +'%Y.%m').01-x86_64.iso" -var "iso_checksum_url=https://downloads.archlinux.de/iso/$(date +'%Y.%m').01/sha1sums.txt"  vagrant.json
+- # ./packer validate  -var "iso_url=https://downloads.archlinux.de/iso/$(date +'%Y.%m').01/archlinux-$(date +'%Y.%m').01-x86_64.iso" -var "iso_checksum_url=https://downloads.archlinux.de/iso/$(date +'%Y.%m').01/sha1sums.txt"  vagrant.json
+- jq 'del(."vagrant-cloud")' vagrant.json | packer validate  -var "iso_url=https://downloads.archlinux.de/iso/$(date +'%Y.%m').01/archlinux-$(date +'%Y.%m').01-x86_64.iso" -var "iso_checksum_url=https://downloads.archlinux.de/iso/$(date +'%Y.%m').01/sha1sums.txt" -
 
 notifications:
   - email: false

vagrant.json

@@ -90,7 +90,8 @@
             "ssh_wait_timeout": "10000s",
             "shutdown_command": "sudo systemctl poweroff",
             "headless": "{{user `headless`}}",
-            "vmx_data": {"memsize": "{{user `memory`}}"},
+            "memory": "{{user `memory`}}",
+            "cpus": "{{user `cpus`}}",
             "boot_command": [
                 "<enter><wait10><wait10><wait10><wait10><wait10><enter><enter>",
                 "curl -O 'http://{{.HTTPIP}}:{{.HTTPPort}}/install{,-chroot}.sh'<enter><wait>",