bat panics on Ruby files with unindented heredocs

When calling `bat` on this Ruby file
``` ruby
<<END_DESC
Test
END_DESC
```
it panics with:
```
thread 'main' panicked at
  'regex string should be pre-tested: Error(-208, invalid backref number/name)',
  …/syntect-4.1.0/src/parsing/regex.rs:70:17
```

found with the Python script in #913.