Use $cmd_open  instead of plain open while opening a new issue in jira.

Here is a bugfix for jira plugin.

I'm using Ubuntu. When I tried opening a new issue by giving "jira" command, I get the following error:

`% jira 
Opening new issue
Couldn't get a file descriptor referring to the console
`

So, when I change the plain "open" command with previously assigned open_cmd variable, this will fix the issue for both linux and Mac's I think.