@import rules with minified CSS that contains other 'url' rules will fail to parse @import's url

Examples include:

````css
@import url("min-import-url-quoted-double.css");.minified{background:url("css/nested/test.png");}

/* Which will incorrectly match the URL of the css file as: */
min-import-url-quoted-double.css");.minified{background:url("css/nested/test.png
`````

````css
@import "min-import-quoted-double.css";.minified{background:url("css/nested/test.png");}

/* Which will incorrectly match the URL of the css file as: */
min-import-quoted-double.css";.minified{background:url("css/nested/test.png
`````

This fix updates the @import regex to fix this issue.

*Note*: See the tests/master/index.html for the unit tests, which have _only been tested in IE8_!