Make compatible with Jekyll 3

[chuckhoupt]

This patch adds a test so the plugin works with both Jekyll 2 and 3.

Thank for this plugin. We use it extensively on a intranet site at FSU Biology's graduate office.

[andyatkinson]

@chuckhoupt Did you get this working with Jekyll 3?

[chuckhoupt]

@andyatkinson Yes my patched version works with Jekyll 3 (just tested with 3.8.4). The patch isn't registered as a Gem, so to use it, put jekyll-markdown-block.rb in the _plugins folder.