Fix mutant loading wrong simplecov-html code

Mutant's bootstrap appends `includes` to `$LOAD_PATH` via `<<`, leaving
the simplecov gem's lib ahead of `./lib`. Since upstream simplecov ships
a backward-compat `lib/simplecov-html.rb` shim, `require
"simplecov-html"` resolved to that shim instead of our own lib — so
mutant introspected and tested simplecov's bundled HTMLFormatter
(different API) instead of this gem's code.

Switching the require to a relative path bypasses `$LOAD_PATH` search
and loads our local file directly.

Author: sferik

.mutant.yml

@@ -6,7 +6,7 @@ includes:
   - test
 requires:
   - simplecov
-  - simplecov-html
+  - ./lib/simplecov-html
 matcher:
   subjects:
     - SimpleCov::Formatter::HTMLFormatter*