Added section about ubuntu dependencies for wagon install

[alexgleason]

I struggled to install wagon on a fresh ubuntu install until realizing I was missing two system dependencies, ruby-dev and g++. After installing them it worked. I installed them with:

sudo apt-get install ruby-dev g++

Just for reference, here is the error I was getting without these packages:

vagrant@vagrant-ubuntu-trusty-64:~$ sudo gem install locomotivecms_wagon
Building native extensions.  This could take a while...
ERROR:  Error installing locomotivecms_wagon:
    ERROR: Failed to build gem native extension.

        /usr/bin/ruby1.9.1 extconf.rb
/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
    from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
    from extconf.rb:2:in `<main>'


Gem files will remain installed in /var/lib/gems/1.9.1/gems/eventmachine-1.0.7 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/eventmachine-1.0.7/ext/gem_make.out