Remove warning about missing file 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
Replace ugly tabs with whitespaces
Merge pull request #86 from infertux/fix_gemspec

Fix gemspec