Use text/javascript, not application/javascript, for .js

[laukstein]

Issue mentioned in github/pages-gem#612

See webhintio/hint#568 (comment),
https://html.spec.whatwg.org/multipage/scripting.html#scriptingLanguages says:

Servers should use text/javascript for JavaScript resources. Servers should not use other JavaScript MIME types for JavaScript resources, and must not use non-JavaScript MIME types.

https://html.spec.whatwg.org/multipage/infrastructure.html#dependencies says:

[dougwilson]

Hi @laukstein ! Our mappings come from the three sources listed at the top of the README. This module just aggregates the sources into a JSON database.

The particular mapping you're referring to (application/javascript -> js) in our database is coming from two sources: the NGINX project and the Apache HTTPD project.

NGINX: http://hg.nginx.org/nginx/raw-file/default/conf/mime.types
Apache HTTPD: http://svn.apache.org/repos/asf/httpd/httpd/trunk/docs/conf/mime.types

If these projects have the incorrect mapping, you'll need to raise the issue in these projects, and once they update their mappings, the updates will automatically be pulled into this database.