Fix bugs when trying to specify rubies in soloistrc without an env hash

When trying to specify rubies in my sprout-wrap soloistrc like this:
    
      sprout:
        rbenv:
          rubies:
            - 2.0.0-p353
            - 1.9.3-p448

… I get the error…

     ================
    Recipe Compile Error in /Users/kfitzpatrick/workspace/sprout-wrap/cookbooks/sprout-osx-rbenv/recipes/default.rb
    =================


    NoMethodError
    -------------
    undefined method `[]' for nil:NilClass


    Cookbook Trace:
    ---------------
      /Users/kfitzpatrick/workspace/sprout-wrap/cookbooks/sprout-osx-rbenv/definitions/ruby_install.rb:15:in `block (2 levels) in from_file'
      /Users/kfitzpatrick/workspace/sprout-wrap/cookbooks/sprout-osx-rbenv/definitions/ruby_install.rb:10:in `block in from_file'
      /Users/kfitzpatrick/workspace/sprout-wrap/cookbooks/sprout-osx-rbenv/recipes/default.rb:10:in `block in from_file'
      /Users/kfitzpatrick/workspace/sprout-wrap/cookbooks/sprout-osx-rbenv/recipes/default.rb:9:in `each'
      /Users/kfitzpatrick/workspace/sprout-wrap/cookbooks/sprout-osx-rbenv/recipes/default.rb:9:in `from_file'


    Relevant File Content:
    ----------------------
    /Users/kfitzpatrick/workspace/sprout-wrap/cookbooks/sprout-osx-rbenv/definitions/ruby_install.rb:

      8:    install_cmd = "#{rbenv_cmd} install #{ruby_version} #{options[:command_line_options]}"
      9:
     10:    execute "installing #{ruby_version} with RBENV: #{install_cmd}" do
     11:      command install_cmd
     12:      user params[:user] || node['current_user']
     13:      only_if params[:only_if] if params[:only_if]
     14:      not_if params[:not_if] || "#{rbenv_cmd} versions | grep #{ruby_version}"
     15>>     env params[:options][:env]
     16:    end
     17:
     18:    execute "check #{ruby_version}" do
     19:      command "#{rbenv_cmd} versions | grep #{ruby_version}"
     20:      user params[:user] || node['current_user']
     21:    end
     22:  end
     23:

This PR solves my problem :)