Allow docker version to be changed

Also test other versions in travis

Author: zmoazeni

.travis.yml

@@ -1,3 +1,5 @@
 language: ruby
 rvm:
   - 2.5
+  - 2.4
+  - 2.3

docker-ruby

@@ -1,2 +1,3 @@
 #!/bin/bash
-docker run -it --rm -v $(pwd):/app -v csscss_gem_data:/usr/local/bundle -w /app ruby $@
+VERSION=${VERSION:-latest}
+docker run -it --rm -v $(pwd):/app -v csscss_gem_data:/usr/local/bundle -w /app ruby:$VERSION $@