added mirror for xdrfile lib on our github organization

.travis/install.xdrfile

@@ -10,8 +10,10 @@ 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
 
-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
 
 tar xzf xdrfile-1.1.4.tar.gz