require: cannot load such file -- kramdown (LoadError)

Reported to me by Sue McClatchy.

~~~
$ make lesson-check
/usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require': cannot load such file -- kramdown (LoadError)
from /usr/local/Cellar/ruby/2.4.1_1/lib/ruby/2.4.0/rubygems/core_ext/kernel_require.rb:55:in `require'
from bin/markdown_ast.rb:5:in `'
Traceback (most recent call last):
File "bin/lesson_check.py", line 565, in
main()
File "bin/lesson_check.py", line 111, in main
docs = read_all_markdown(args.source_dir, args.parser)
File "bin/lesson_check.py", line 214, in read_all_markdown
data = read_markdown(parser, filename)
File "/Users/smc/Projects/Funding/BD2K_OER/ngs-lesson/bin/util.py", line 120, in read_markdown
doc = json.loads(stdout_data)
File "//anaconda/lib/python3.5/json/__init__.py", line 319, in loads
return _default_decoder.decode(s)
File "//anaconda/lib/python3.5/json/decoder.py", line 339, in decode
obj, end = self.raw_decode(s, idx=_w(s, 0).end())
File "//anaconda/lib/python3.5/json/decoder.py", line 357, in raw_decode
raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
make: *** [lesson-check] Error 1
~~~