eslint: Fix invalid filename validation

Attempt to fix the following error which is appearing in every new MR since eslint-plugin-filenames was introduced.

/builds/gitlab-org/gitlab-ce/app/assets/javascripts/gl_field_error.js.es6
  2:1  error  Filename 'gl_field_error.js.es6' does not match the naming convention  filenames/match-regex

This was happening because our ES6 files have the following structure gl_hello_world.js.es6. So eslint-plugin-filenames was considering as base name gl_hello_world.js which didn't pass the previous regex ^[a-z_]+$

This new regex allows the following filenames to pass as valid:

• gl_hello_world.js.es6 which base name translates to gl_hello_world.js
• gl_foo_bar.js which base name translates to gl_foo_bar
• d3.js which base name translates to d3 (especial case, maybe we should move this to the vendor dir and make the regex more specific)