Fixed publication links in README.md.
Fixed publication links in README.md.

SheffieldML/GPmat,TianpeiLuke/GPmat,SheffieldML/GPmat,SheffieldML/GPmat,TianpeiLuke/GPmat,TianpeiLuke/GPmat,TianpeiLuke/GPmat,TianpeiLuke/GPmat,SheffieldML/GPmat,SheffieldML/GPmat,SheffieldML/GPmat,TianpeiLuke/GPmat