need to match builder version of activemodel

Test suite seems to pass. CI will tell for sure. 

Before:

```
$ bundle
...
Bundler could not find compatible versions for gem "builder":
  In Gemfile:
    mongoid (~> 4.0.0.alpha2) ruby depends on
      builder (~> 3.1.0) ruby

    grape (>= 0) ruby depends on
      builder (3.2.2)
```

After:

```
$ bundle
...
Your bundle is complete!
Use `bundle show [gemname]` to see where a bundled gem is installed.
```