update ubuntu

[HungUnicorn]

Got checksum error again and again, and updating Ubuntu works.

C:\Users\user\devops\packer-templates>packer build -only=virtualbox-iso application-server.json
virtualbox-iso output will be in this color.

==> virtualbox-iso: Downloading or copying Guest additions
    virtualbox-iso: Downloading or copying: file:///C:/Program%20Files/Oracle/VirtualBox/VBoxGuestAdditions.iso
==> virtualbox-iso: Downloading or copying ISO
    virtualbox-iso: Downloading or copying: http://releases.ubuntu.com/trusty/ubuntu-14.04.4-server-amd64.iso
    virtualbox-iso: Error downloading: checksums didn't match expected: 07e4bb5569814eab41fafac882ba127893e3ff0bdb7ec931c9b2d040e3e94e7a
==> virtualbox-iso: ISO download failed.
Build 'virtualbox-iso' errored: ISO download failed.

==> Some builds didn't complete successfully and had errors:
--> virtualbox-iso: ISO download failed.

==> Builds finished but no artifacts were created.

[julialamenza]

works for me too

[lucasnorman]

Worked for me as well.

[abhishekjt]

Worked for me as well 👍

[13rac1]

Duplicate of #28