fix: Add Ruby icon to badge, upcase Ruby, add Mexico flag.
fix: Add Ruby icon to badge, upcase Ruby, add Mexico flag.

1. Why is this change neccesary?
Because:
 - The badge was missing a Ruby icon.
 - Ruby was writen with lowercase when it should be written uppercase.
 - There were some typos in the README.
 - The Mexico flag gives a nice look to the README.

2. How does it address the issue?
Everything was added.

3. What side effects does this change have?
None.

conekta/conekta-ruby