Error installing nokogiri gem with rbenv ruby 2.2.1

I'm trying to install a fresh nokogiri gem on a rails app like so:

gem install nokogiri -v '1.4.4'

I'm getting this error:

Building native extensions.  This could take a while...
ERROR:  Error installing nokogiri:
    ERROR: Failed to build gem native extension.

    /Users/development/.rbenv/versions/2.2.1/bin/ruby -r ./siteconf20150331-4057-14pcvq9.rb extconf.rb
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.  Check the mkmf.log file for more details.  You may
need configuration options.

Provided configuration options:
    --with-opt-dir
    --without-opt-dir
    --with-opt-include
    --without-opt-include=${opt-dir}/include
    --with-opt-lib
    --without-opt-lib=${opt-dir}/lib
    --with-make-prog
    --without-make-prog
    --srcdir=.
    --curdir
    --ruby=/Users/development/.rbenv/versions/2.2.1/bin/$(RUBY_BASE_NAME)
extconf.rb:10:in `<main>': uninitialized constant Config (NameError)

extconf failed, exit code 1

Gem files will remain installed in /Users/development/.rbenv/versions/2.2.1/lib/ruby/gems/2.2.0/gems/nokogiri-1.4.4 for inspection.
Results logged to /Users/development/.rbenv/versions/2.2.1/lib/ruby/gems/2.2.0/extensions/x86_64-darwin-14/2.2.0-static/nokogiri-1.4.4/gem_make.out

I'm using rebenv ruby 2.1.1, so this is in a gemset.

Also this might help, this is my brew list:

autoconf    freexl      gpp     libpng      lzlib       postgis     redis
automake    gdal        imagemagick libspatialite   makedepend  postgresql  ruby-build
boost       geoip       jpeg        libtiff     mpfr        proj        s3cmd
cgal        geos        json-c      libtool     openssl     qt      sfcgal
cmake       giflib      libgeotiff  libxml2     phantomjs   rbenv       sqlite
freetype    gmp     liblwgeom   libxslt     pkg-config  readline    xz

Thanks for your help, I could really do with this working, there's a lot of similar fixes online however none of them have fixed my issue.