Fix gemspec

`s/CHANGELOG/CHANGELOG.md` to avoid this when installing the Gem:

```
rack-mini-profiler at .../rack-mini-profiler did not have a valid gemspec.
    This prevents bundler from installing bins or native extensions, but that may not affect its functionality.
    The validation message from Rubygems was:
      ["CHANGELOG"] are not files
```

(also replaced all tabs with spaces)