Build docker image for running tests

Published at `ericfreese/zsh-autosuggestions-test`.

Contains:
- Ruby 2.5.3
- Supported zsh versions 4.3.11 through 5.6.2
- Test ruby dependencies

Also updates Circle CI config to use built docker image.

install_test_zsh.sh

@@ -0,0 +1,26 @@
+#!/bin/sh
+
+set -ex
+
+for v in $(grep "^[^#]" ZSH_VERSIONS); do
+  mkdir zsh-$v
+  cd zsh-$v
+
+  curl -L https://api.github.com/repos/zsh-users/zsh/tarball/zsh-$v | tar xz --strip=1
+
+  ./Util/preconfig
+  ./configure --enable-pcre \
+              --enable-cap \
+              --enable-multibyte \
+              --with-term-lib='ncursesw tinfo' \
+              --without-tcsetpgrp \
+              --program-suffix="-$v"
+
+  make install.bin
+  make install.modules
+  make install.fns
+
+  cd ..
+
+  rm -rf zsh-$v
+done