add comment why packer's version is hardcoded

As of writing there is no 'latest' (hashicorp/packer/issues/5265)

add comment why packer's version is hardcoded
14754c44 · Kilian Engelhardt · 99062dcf · 14754c44
Unverified Commit 14754c44 authored 7 years ago by Kilian Engelhardt
--- a/.travis.yml
+++ b/.travis.yml
@@ -6,6 +6,7 @@ env:
  matrix:
  - DIST=trusty
  global:
+    # 20170821: as of writing there is no 'latest' (hashicorp/packer/issues/5265)
  - PACKER_ZIP="packer_1.0.4_linux_amd64.zip"
  - PACKER_URL="https://releases.hashicorp.com/packer/1.0.4/packer_1.0.4_linux_amd64.zip"