Fix css compile issue in build mode

when I was fixing css path issue in _branch fix-css-path-issue-in-build-mode_, I didn't check it in UAT server. Actually, I missed something that the *useminPrepare* job would not update the html file to target.

when we are doing the css and js minification operation using *useminPrepare* and *usemin*, we expect
```jade
// build:css /dist/css/main.css
link(type="text/css", rel="stylesheet", href="/bower_components/Font-Awesome/css/font-awesome.css")
link(type="text/css", rel="stylesheet", href="/bower_components/pure/pure.css")
link(type="text/css", rel="stylesheet", href="/css/main.css")
// endbuild
```
to be compiled to 
```jade
link(type="text/css", rel="stylesheet", href="/css/main.css")
```

but the template is a backend template, which should be compiled from backend, so add condition
```js
if process.env.NODE_ENV === 'dev' || front_end_build === true
```
to verify if it should be compiled.