Don't use let or const

[kratiahuja]

It is breaking ember-cli-fastboot tests. Fixes: #4927

cc: @runspired @stefanpenner @hjdivad

[runspired]

Thanks @kratiahuja. I forget we can't have nice things ;)

[kratiahuja]

On that note, why is ember-data only being tested against Node 6 (I thought ember-cli supports Node 4)? Travis config shows only Node 6

[runspired]

@bmac seems to have introduced the node 6 change.

[bmac]

I changed travis to use node 6 because master was broken during the contributor workshop and I wanted to unblock contributors. Sounds like I need to revert back to node 4 and debug the real issue.

[rwjblue]

let and const are fine in node 4, you just have to ensure 'use strict'; is at the top of the file.

[rwjblue]

@bmac - the original reason there was a failure around the time of EmberConf was that a new version of ember-try introduced some issues with npm@2 (which is used by node 4). That was fixed within a couple days of EmberConf, so it should be good now...