Tweak README

Changes include:

* Stressing that allowing CORS from **any** domain is dangerous

* Switching to shields.io SVG badges

* Fixing whitespace

* Fixing OCD stuff
Specify Ruby 1.9.3 and 2.1 in Travis config

Since Ruby 2.1.0, the MRI versioning policy changed to follow Semantic
Versioning [1].

Thus, it is better to use '2.1' instead of '2.1.x' in Travis config
since this way the latest patch release (at this point 2.1.2) will be
selected automatically.

[1]:
https://www.ruby-lang.org/en/news/2013/12/21/ruby-version-policy-changes-with-2-1-0/
Remove unused folders and files
Fix typo in the gemspec