Replace leading whitespace in commit message.

This allows for proper indentation to display ASCII art.

Fixes travis-ci/travis-ci#2003
Set whitespace handling via CSS.

Relevant for travis-ci/travis-ci#2003.
Wrap commit body into a pre tag.

This follows the idea that the message can contain useful things
like ASCII diagrams or wonderful art.

Related to travis-ci/travis-ci#2003.
Decrease line height for commit body slightly.

18px seems like a good compromise between ASCII art and
readability.
Show xcode_sdk and scheme as matrix dimensions.

Fixes travis-ci/travis-ci#2177.