replaced xdrfile ftp

xdrfile source has been removed from gromacs ftp

I put a copy on people.sissa.it/~bussi/plumed so that we
can download it on travis

.travis/install.xdrfile

@@ -4,7 +4,11 @@ set -e
 set -x
 
 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
 tar xzf xdrfile-1.1.4.tar.gz
 cd xdrfile-1.1.4
 ./configure --enable-shared --prefix=/usr/local