Merge branch 'v2.4' into v2.5

.travis/install.xdrfile

@@ -6,12 +6,11 @@ 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