Revert "official ftp for xdrfile"

This reverts commit 1486c337.

.travis/install.xdrfile

@@ -6,11 +6,12 @@ set -x
 cd "$(mktemp -dt plumed.XXXXXX)"
 
 echo "installing xdrfile library"
-wget ftp://ftp.gromacs.org/pub/contrib/xdrfile-1.1.4.tar.gz
+# wget ftp://ftp.gromacs.org/pub/contrib/xdrfile-1.1.4.tar.gz
 # 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
 
 tar xzf xdrfile-1.1.4.tar.gz