Revert "added mirror for xdrfile lib on our github organization"

This reverts commit f0cc9dcd.

.travis/install.xdrfile

@@ -10,10 +10,8 @@ echo "installing xdrfile library"
 # xdrfile was removed from gromacs ftp
 # as a workaround I added a copy to people.sissa.it/~bussi/plumed
 # this complies with its license
-# wget people.sissa.it/~bussi/plumed/xdrfile-1.1.4.tar.gz
 
-# better to download from github, that should be even more reliable:
-wget https://github.com/plumed/other_packages/blob/master/xdrfile-1.1.4.tar.gz
+wget people.sissa.it/~bussi/plumed/xdrfile-1.1.4.tar.gz
 
 tar xzf xdrfile-1.1.4.tar.gz