Update .ruby-version

I have included this cookbook as a dependency in a wrapper cookbook. I am trying to use Berkshelf 3.1.3 to run a `berks upload` to upload my wrapper cookbook and all of it's dependencies to the Chef Server. However, when Berkshefl gets to this cookbook it spits out an error:

```
E, [2014-09-23T17:59:24.582763 #12389] ERROR -- : Cookbook file attributes/default.rb has a ruby syntax error:
E, [2014-09-23T17:59:24.582875 #12389] ERROR -- : rbenv: version `1.9.3' is not installed
E, [2014-09-23T17:59:24.582908 #12389] ERROR -- : rbenv: version `1.9.3' is not installed
E, [2014-09-23T17:59:24.586324 #12389] ERROR -- : Ridley::Errors::CookbookSyntaxError: Invalid ruby files in cookbook: postgresql (0.14.1).
E, [2014-09-23T17:59:24.587552 #12389] ERROR -- : /opt/chefdk/embedded/lib/ruby/gems/2.1.0/gems/ridley-4.0.0/lib/ridley/chef/cookbook.rb:198:in `validate'
```

I manually opened up the cookbook that Berkshelf had in it's cache, and updated the `.ruby-version` file to one of the version that I had available in the `rbenv` repo (in my case 1.9.3-p545), and Berkshelf stopped complaining.

It odes not appear that there is a plain 1.9.3 version even available in the rbenv repo. Here are all the 1.9.3 builds available:

```
  1.9.3-dev
  1.9.3-p0
  1.9.3-p125
  1.9.3-p194
  1.9.3-p286
  1.9.3-p327
  1.9.3-p362
  1.9.3-p374
  1.9.3-p385
  1.9.3-p392
  1.9.3-p429
  1.9.3-p448
  1.9.3-p484
  1.9.3-p545
  1.9.3-preview1
  1.9.3-rc1
```

If this cookbook is going to lock itself to 1.9.3, perhaps it should be locked to a build that is available via rbenv