Guarantee correct Ruby/Jekyll compatibility, for RVM users

If you don't specify using >1.9.3 globally with RVM, jekyll commands don't work until you type `$ rvm use 1.9.3` first. Having the .rvmrc file just does this for you.

Note: it will produce a trust prompt on the first `$ cd` into the directory, that looks like this:

```
$ cd simpleA11y/
==============================================================================
= NOTICE                                                                     =
==============================================================================
= RVM has encountered a new or modified .rvmrc file in the current directory =
= This is a shell script and therefore may contain any shell commands.       =
=                                                                            =
= Examine the contents of this file carefully to be sure the contents are    =
= safe before trusting it! ( Choose v[iew] below to view the contents )      =
==============================================================================
Do you wish to trust this .rvmrc file? (/path/to/the/file/.rvmrc)
y[es], n[o], v[iew], c[ancel]>
```