wget -qO - https://packages.elasticsearch.org/GPG-KEY-elasticsearch | sudo apt-key add -
echo "deb http://packages.elasticsearch.org/elasticsearch/1.4/debian stable main" | sudo tee -a /etc/apt/sources.list
apt-get update && apt-get install elasticsearch
I do not yet know if this is enough to have a fully working installation of Elasticsearch (but I'm ready to bet it isn't), but in this way at least the installation completes successfully.
LONG ANSWER
These are their instruction as for today:
To begin, add-apt-repository is not installed by default in both Ubuntu 12.04 and 14.04. To use this tool you need to install python-software-properties:
sudo apt-get install python-software-properties
Then you can execute the second command of the official instructions, which is:
sudo add-apt-repository "deb http://packages.elasticsearch.org/elasticsearch/1.4/debian stable main"
But this command has the ugly side effect below when you run apt-get update:
This error should be fairly unhurting, and anyway now you can run apt-get install elasticsearch and install it successfully.
But, these instruction introduced a probably unnecessary software and an error in you system, even if unharmful. And in my opinion a serious company should avoid in any way those things, especially in the page where people could become your customers. But, hey, I'm the tech guy, not in marketing or management.
And below you can find what introduced this error:
