fix a bug in the ash-template

[stew]

[ "foo" == "bar" ] is not valid posix. It should be a single =, or else you'll get "unexpected operator/operand" on a posix compliant shell:

> posh
$ [ "foo" = "foo" ] && echo yes 
yes
$ [ "foo" == "foo" ] && echo yes
posh: [: ==: unexpected operator/operand

> dash                                                                                                               ⏎
$ [ "foo" = "foo" ] && echo yes 
yes
$ [ "foo" == "foo" ] && echo yes
dash: 2: [: foo: unexpected operator

[lightbend-cla-validator]

Hi @stew,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Lightbend Contributors License Agreement:

http://www.lightbend.com/contribute/cla

[muuki88]

Thanks. LGTM. I'll keep this open until two weeks and then release it immediately (also for the 1.x series)