Update formatter to handle older Elixir versions

[smpoulsen]

18e7b99 updated the formatter to handle ExUnit's tests_counter change from an integer to a map in https://github.com/elixir-lang/elixir/commit/c4d6be64581b9587dafe4ea5aef346e1684461da#diff-0ed5c47cc55f463c3af34af96d1fef62L18 in a way that wasn't backwards compatible with older Elixir versions.

This updates the formatter to handle older Elixir versions by checking whether tests_counter is a map and updating it accordingly.

[coveralls]

Coverage increased (+0.9%) to 73.864% when pulling 3ccec2818830bdc830708c7dad7008abba2977af on tpoulsen:handle-old-elixir-versions-in-formatter into 5370647409e0276dc5f0aba5289ac623993460fb on parroty:master.

[coveralls]

Coverage increased (+0.6%) to 73.563% when pulling 03eabc423990d148e9ab9ffd2e1797d7bd6ccc66 on tpoulsen:handle-old-elixir-versions-in-formatter into 5370647409e0276dc5f0aba5289ac623993460fb on parroty:master.