Remove --color option from .rspec

The `--color` option is not project-specific option, but for each user's
preferences. There is no meaning to force colorized output in this project.
If you'd like to pass extra options for your preferences to RSpec like
--format and --color and so on, write them into your ~/.rspec file.

Author: r7kamura

.rspec

@@ -1,2 +1 @@
---color
 --require spec_helper