errors installing gem "rumember" in Ubuntu 16.4 LTS - ruby

Question

I tried for several hours to get the rumember gem installed and running.
After a failed upgrade of my system to 16.4 Ubuntu LTS i had to reinstall the system completely (with 16.4. But now I cant get the rumember gem to work again.
I googled a lot and tried several approaches. e.g. I installed the ruby-dev package for headers and I ran sudo gem update --system
and got "Latest version currently installed ..."
I see that the json version in the errors is 1.7.7 and in my system its 1.8.3. but i don't know if that matters.
Thats how far I got. Thanks for any Ideas on the topic.
Update 1: I just tried to install it on a fresh VM Ubuntu LTS 16.4. Install ruby, install ruby-dev, gem install rumember. Same problems occur. So it's not just my setup.
sudo gem install rumember
outputs:
ERROR: Error installing rumember:
ERROR: Failed to build gem native extension.
current directory: /var/lib/gems/2.3.0/gems/json-1.7.7/ext/json/ext/generator
/usr/bin/ruby2.3 -r ./siteconf20161214-5373-1y3th00.rb extconf.rb
creating Makefile
current directory: /var/lib/gems/2.3.0/gems/json-1.7.7/ext/json/ext/generator
make "DESTDIR=" clean
current directory: /var/lib/gems/2.3.0/gems/json-1.7.7/ext/json/ext/generator
make "DESTDIR="
compiling generator.c
In file included from generator.c:1:0:
../fbuffer/fbuffer.h: In function ‘fbuffer_to_s’:
../fbuffer/fbuffer.h:175:47: error: macro "rb_str_new" requires 2 arguments, but only 1 given
VALUE result = rb_str_new(FBUFFER_PAIR(fb));
^
../fbuffer/fbuffer.h:175:20: warning: initialization makes integer from pointer without a cast [-Wint-conversion]
VALUE result = rb_str_new(FBUFFER_PAIR(fb));
^
Makefile:239: recipe for target 'generator.o' failed
make: *** [generator.o] Error 1
make failed, exit code 2
Gem files will remain installed in /var/lib/gems/2.3.0/gems/json-1.7.7 for inspection.
Results logged to /var/lib/gems/2.3.0/extensions/x86_64-linux/2.3.0/json-1.7.7/gem_make.out
gem list outputs:
bigdecimal (default: 1.2.8)
bundler (1.11.2)
configuration (1.3.4)
did_you_mean (1.0.0)
diff-lcs (1.2.5)
io-console (default: 0.4.5)
json (default: 1.8.3)
launchy (0.3.7)
minitest (5.8.4)
molinillo (0.4.3)
net-http-persistent (2.9.4)
net-telnet (0.1.1)
power_assert (0.2.7)
psych (default: 2.0.17)
rake (10.5.0)
rdoc (default: 4.2.1)
rspec (3.5.0)
rspec-core (3.5.4)
rspec-expectations (3.5.0)
rspec-mocks (3.5.0)
rspec-support (3.5.0)
rubygems-update (2.0.3)
test-unit (3.1.7)
thor (0.19.1)