ci: build SBCL from source.

Travis and Netlify both use Ubuntu 16 as their build distro. The
version of SBCL available in Ubuntu 16 is `1.3.1` which contains
`asdf` version `3.1.5`. `launch-program` was added in `asdf`
version `3.1.7.26`. The absence of `launch-program` means
`3bmd-ext-code-blocks` will fail to build. To address that I
decided to build SBCL from source in Travis to get a more up to date
version of `asdf` (which is bundled with `sbcl`).

This turned out to be a lovely yak shaving exercise.

.travis.yml

@@ -1,19 +1,13 @@
 language: lisp
 sudo: required
 
-dist: focal
+dist: xenial
 os:
   - linux
   - osx
 
 install:
-  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo apt-get update -qq; fi
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update; fi
-  - if [[ "$TRAVIS_OS_NAME" == "linux" ]]; then sudo apt-get install zlib1g-dev sbcl; fi
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew install sbcl; fi
-  - curl -O "https://beta.quicklisp.org/quicklisp.lisp"
-  - sbcl --non-interactive --load quicklisp.lisp --eval "(quicklisp-quickstart:install :path \"~/quicklisp\")"
-  - sbcl --non-interactive --load ~/quicklisp/setup.lisp --eval "(ql::without-prompting (ql:add-to-init-file))"
+  - ./ci/install.sh
 
 script:
   - make cycle

ci/install.sh

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+if [ "$TRAVIS_OS_NAME" = "linux" ]; then
+    sudo apt update -qy;
+    sudo apt install -qy curl zlib1g-dev sbcl build-essential git;
+    git clone --depth 1 -b sbcl-2.0.7 git://git.code.sf.net/p/sbcl/sbcl /tmp/sbcl;
+    cd /tmp/sbcl || exit;
+    sh make.sh --with-sb-core-compression;
+    sudo sh install.sh;
+    sudo apt remove sbcl;
+fi
+
+if [ "$TRAVIS_OS_NAME" = "osx" ]; then
+    brew update;
+    brew install sbcl;
+fi
+
+curl -O "https://beta.quicklisp.org/quicklisp.lisp";
+/usr/local/bin/sbcl --non-interactive --load quicklisp.lisp --eval "(quicklisp-quickstart:install :path \"~/quicklisp\")";
+/usr/local/bin/sbcl --non-interactive --load ~/quicklisp/setup.lisp --eval "(ql::without-prompting (ql:add-to-init-file))";