upgrade mongoid 6.0 to support ruby2.4

[machu]

ruby2.4.0ではmongoid 5系が動かないので、6系にアップグレードしました。

動かない理由はmongoid5系がjson1.8.3に間接的に依存しており、json1.8.3がruby2.4.0に対応していないためです。 ruby/json#303 によると次のRails 4.2.8で対応するようですが、それをまたずにmongoidを上げてしまおうかと。

$ ebenv local 2.4.0
$ bundle
Fetching gem metadata from https://rubygems.org/........
Fetching version metadata from https://rubygems.org/.
Resolving dependencies...
（中略）
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

    current directory: /Users/machu/.rbenv/versions/2.4.0/lib/ruby/gems/2.4.0/gems/json-1.8.3/ext/json/ext/generator
/Users/machu/.rbenv/versions/2.4.0/bin/ruby -r ./siteconf20161229-75425-gs1efs.rb extconf.rb
creating Makefile

current directory: /Users/machu/.rbenv/versions/2.4.0/lib/ruby/gems/2.4.0/gems/json-1.8.3/ext/json/ext/generator
make "DESTDIR=" clean

current directory: /Users/machu/.rbenv/versions/2.4.0/lib/ruby/gems/2.4.0/gems/json-1.8.3/ext/json/ext/generator
make "DESTDIR="
compiling generator.c
generator.c:861:25: error: use of undeclared identifier 'rb_cFixnum'
    } else if (klass == rb_cFixnum) {
                        ^
generator.c:863:25: error: use of undeclared identifier 'rb_cBignum'
    } else if (klass == rb_cBignum) {
                        ^
2 errors generated.
make: *** [generator.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/machu/.rbenv/versions/2.4.0/lib/ruby/gems/2.4.0/gems/json-1.8.3 for inspection.
Results logged to /Users/machu/.rbenv/versions/2.4.0/lib/ruby/gems/2.4.0/extensions/x86_64-darwin-15/2.4.0-static/json-1.8.3/gem_make.out

An error occurred while installing json (1.8.3), and Bundler cannot continue.
Make sure that `gem install json -v '1.8.3'` succeeds before bundling.

[tdtds]

これで行ってみましょう。マージします。