Revert "Install coveralls"

This reverts commit c64eba2ef563c33c2e862ec0cfdf1c008bd01eb3.

Coveralls depends on the "colorize" gem:
https://github.com/lemurheavy/coveralls-ruby/blob/2081d0382b7bd8796634d39d1c06f65241dac836/coveralls-ruby.gemspec#L19

This gem's monkey patches of Ruby's String class override the colored gem's
monkey patches and break the build.