Rename Google Analytics variables

`b` `o` `i` `l` `e` `r` -> `c` `s` `S` `n` `e` `x` `t`

It is generated by [isogram](https://github.com/shinnn/isogram), a google analytics code generator used by [gulp website](https://github.com/gulpjs/gulpjs.github.io/blob/fc2a237ef4b476dd9428fce0da77c6bd3f8b925d/index.html#L134).