Add missing main Ruby file

[RKushnir]

Addresses https://github.com/daxtens/fast_secure_compare/issues/2

[antonpaisov]

:+1:

[daxtens]

This is causing the Travis tests to fail --- can you address that please?

[RKushnir]

@daxtens I'll have a look

[RKushnir]

@daxtens So I made the tests pass, but this is confusing. Now I think the code should have been working even without this pull request, as it just adds some redirection - instead of requiring the *.so file directly we now require it through *.rb file.

upd. Actually, it put the *.so file at lib/fast_secure_compare before, but when I added fast_secure_compare.rb it started putting the *.so file directly in the lib.