Update README to include gem requirement

[pbootly]

Closes #3

Whilst likely obvious to people from the error thrown:

Traceback (most recent call last):
        2: from ./redis-sampler.rb:31:in `<main>'
        1: from /usr/lib/ruby/vendor_ruby/rubygems/core_ext/kernel_require.rb:85:in `require'
/usr/lib/ruby/vendor_ruby/rubygems/core_ext/kernel_require.rb:85:in `require': cannot load such file -- redis (LoadError)

seemed worth calling out the specific install required.