Section subscription modified, a small design

## Section subscription modified, a small design

Changes:
- Size font resized to 1.53em
- The label changed to "Súmate a la comunidad de datos abiertos de México." 
- Deleted the css extra classes in the button "Suscríbeme"


### Screenshot

![bienvenida codeando mexico - 3](https://cloud.githubusercontent.com/assets/660973/3695125/7da32f78-137a-11e4-9450-71d9dc863ddc.png)


Signed-off-by: Miguel Angel Gordian <miguel.angel@codeandomexico.org>