Prevent ghrel from downloading prerelease

[crazywhalecc]

Fix #239 .

[DubbleClick]

Maybe make sure that tags also don't contain 'beta' or 'rc', although this may be complicated.

[crazywhalecc]

@DubbleClick libpng GitHub repo tag cannot get stable version, we still need hard-coded url to download it.

https://api.github.com/repos/glennrp/libpng/tags

It seems that the https://git.code.sf.net/p/libpng/code we are using now is the stable branch of libpng16.

[DubbleClick]

@DubbleClick libpng GitHub repo tag cannot get stable version, we still need hard-coded url to download it.

https://api.github.com/repos/glennrp/libpng/tags

It seems that the https://git.code.sf.net/p/libpng/code we are using now is the stable branch of libpng16.

If you check in the files it's downloading, it pulls the 1.6.41 version which is not released yet. 1.6.40 is the current stable version which is tagged on github/sourceforge.

Weird that the api returns all the 17.x versions. The normal link shows the 1.6.x tags: https://github.com/glennrp/libpng/tags