Fixes #207: Updated docs to require ruby 2.3.3 [ci-skip]

Signed-off-by: Alex Ives <alex@ives.mn>

doc/prepare.md

@@ -9,7 +9,7 @@ during installation.
 1. A non-root Unix user, this can be your normal user but **DO NOT** run the
    installation as a root user
-1. Ruby 2.3 (2.3.1 or newer) installed with a Ruby version manager
+1. Ruby 2.3 (2.3.3 or newer) installed with a Ruby version manager
    ([RVM](https://rvm.io/), [rbenv], [chruby], etc.), **DO NOT** use the
    system Ruby
 1. Bundler, which you can install with `gem install bundler`