(Doc) Chef 10.16.4 install fails w/ Ruby 2.2.2 due to json gem

Some users attempting to install this exact version combination may run into a problem where the json gem is failing to build. For example:

```
gem install chef --version="10.16.4"
[snip]
Fetching: json-1.6.1.gem (100%)
Building native extensions. This could take a while...
ERROR: Error installing chef:
ERROR: Failed to build gem native extension.

/home/jaustinhughey/.rbenv/versions/2.2.2/bin/ruby -r ./siteconf20150529-1424-1rmvbxh.rb extconf.rb
checking for ruby/re.h... yes
checking for ruby/encoding.h... yes
creating Makefile

make "DESTDIR=" clean

make "DESTDIR="
compiling generator.c
In file included from generator.c:1:0:
generator.c: In function ‘cState_aref’:
generator.h:134:27: warning: variable ‘state’ set but not used [-Wunused-but-set-variable]
JSON_Generator_State *state; \
^
generator.c:736:5: note: in expansion of macro ‘GET_STATE’
GET_STATE(self);
^
generator.c: In function ‘fbuffer_to_s’:
generator.c:952:47: error: macro "rb_str_new" requires 2 arguments, but only 1 given
VALUE result = rb_str_new(FBUFFER_PAIR(fb));
^
generator.c:952:20: warning: initialization makes integer from pointer without a cast
VALUE result = rb_str_new(FBUFFER_PAIR(fb));
^
Makefile:237: recipe for target 'generator.o' failed
make: *** [generator.o] Error 1

make failed, exit code 2

Gem files will remain installed in /home/jaustinhughey/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/gems/json-1.6.1 for inspection.
Results logged to /home/jaustinhughey/.rbenv/versions/2.2.2/lib/ruby/gems/2.2.0/extensions/x86_64-linux/2.2.0-static/json-1.6.1/gem_make.out
```

In my case I was able to fix this with:

```
rbenv install 2.1.6
rbenv global 2.1.6
gem install bundler json
rbenv rehash
gem install chef --version="10.16.4"
```

Note, of course, I'm using rbenv. Modify those commands to suit your Ruby environment manager of choice, or don't forget `sudo` if you're rolling raw.