travis: use libressl v3.1.4 instead of master

... as their git master seems too fragile to use (and 3.2.1 which is the
latest has a build failure).

Closes #5964

scripts/travis/before_script.sh

@@ -100,7 +100,7 @@ fi
 
 if [ "$TRAVIS_OS_NAME" = linux -a "$LIBRESSL" ]; then
   cd $HOME
-  git clone --depth=1 https://github.com/libressl-portable/portable.git libressl-git
+  git clone --depth=1 -b v3.1.4 https://github.com/libressl-portable/portable.git libressl-git
   cd libressl-git
   ./autogen.sh
   ./configure --prefix=$HOME/libressl