Avoiding non-mainline GLFW releases to appear on downloads page

Previous code was always looking for latest post tagged `changelog` title to update and provide download links on downloads page. This commit fixes a bug resulting non-mainline GLFW 2.x bug fix releases to appear on downloads page, by forcing mainline GLFW 3.x releases to only show up.

Current code looks for latest post tagged as `changelog` which its title first character contains 3. Whenever GLFW reaches 4.x series, it should be changed to compare the first character with 4; And whenever GLFW major version string contains more than one character (GLFW 10.x), it should be changed to return first 2 characters of post title and compare it with 10.

And Happy GLFW 3.0 release :smile: