Add pandoc2 support to travis/tox

- Install pandoc2 in travis and decide whether to use it or not in tox.
- A symbolic link is created in tox, due to not being able to make
  setenv work like this: `setenv = PATH={toxinidir}:{env:PATH}`
- Install pandoc2 only on pyxx-pandoc2
- Add new inclusion to travis matrix combination
- A build failed with InsecurePlatformWarning on Travis.

scripts/install-pandoc2.sh

@@ -0,0 +1,25 @@
+#!/bin/sh
+
+set -ex
+
+PANDOC_VER=2.2.1
+PANDOC_TGZ=pandoc-${PANDOC_VER}-linux.tar.gz
+PARENT_DIR=${1:-.}
+PANDOC_BIN=${PARENT_DIR}/pandoc2/bin/pandoc
+
+if [ -x "$PANDOC_BIN" ]; then
+    echo "pandoc2 already installed"
+    exit 0
+fi
+
+cd "$PARENT_DIR"
+
+wget "https://github.com/jgm/pandoc/releases/download/${PANDOC_VER}/${PANDOC_TGZ}" -O "${PANDOC_TGZ}"
+
+tar -xzvf "${PANDOC_TGZ}"
+
+rm -rf pandoc2
+
+mv "pandoc-${PANDOC_VER}" pandoc2
+
+[ -x "$PANDOC_BIN" ] || exit 1