improve install instructions on README.md

[weakish]

If a Debian/Ubuntu user have not installed ruby1.9-dev before, when they use gem install gist, they will get this error:

$ sudo gem install gist
Building native extensions.  This could take a while...
ERROR:  Error installing gist:
        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:1:in `<main>'

This is because mkmf.rb is missing on their system.

Thus they need to install mkmf.rb, which is included in the ruby1.9-dev package.

After that, gem install gist works.

[ConradIrwin]

Thanks! I've actualy fixed this by removing the dependency on the JSON gem instead as:

1. json is in the standard library in ruby 1.9 and above...
2. we already have vendor/json for use with homebrew...

Should be fixed in 4.1.3, would love confirmation :).

[weakish]

@ConradIrwin I tested it on a virtual machine (Debian 7). It works!