travis/prepare: use lz4 compression type for pkgs/repos.

lz4 is the fastest one available with libarchive, and
size is not the most important thing in travis.

common/travis/prepare.sh

@@ -1,6 +1,6 @@
 #!/bin/sh
 #
-# install_tools.sh
+# prepare.sh
 
 [ "$XLINT" ] && exit 0
 
@@ -10,6 +10,8 @@ echo XBPS_ALLOW_RESTRICTED=yes >> etc/conf
 /bin/echo -e '\x1b[32mEnabling ethereal chroot-style...\x1b[0m'
 echo XBPS_CHROOT_CMD=ethereal >> etc/conf
 echo XBPS_ALLOW_CHROOT_BREAKOUT=yes >> etc/conf
+echo XBPS_PKG_COMPTYPE=lz4 >> etc/conf
+echo XBPS_REPO_COMPTYPE=lz4 >> etc/conf
 
 /bin/echo -e '\x1b[32mLinking / to /masterdir...\x1b[0m'
 ln -s / masterdir