Add activerecord-explain-analyze gem

This gem allows you to get the `EXPLAIN ANALYZE` query plan, directly
from the Rails console.

The gem is installed with `require: false`, but if it was loaded on
launch, this would be it's memory load:

```
TOP: 145.3086 MiB
  rails/all: 22.4844 MiB
  ...
  activerecord-explain-analyze: 2.9648 MiB
    active_record/connection_adapters/postgresql_adapter: 2.9648 MiB
      pg: 2.9648 MiB
        pg_ext: 2.9648 MiB
  ...
```

1 files changed, 1 insertions, 0 deletions

diff --git a/Gemfile b/Gemfile
index 66348414506..55e1bd58a38 100644
--- a/Gemfile
+++ b/Gemfile
@@ -403,6 +403,7 @@ gem 'ruby-prof', '~> 0.17.0'
 gem 'rbtrace', '~> 0.4', require: false
 gem 'memory_profiler', '~> 0.9', require: false
 gem 'benchmark-memory', '~> 0.1', require: false
+gem 'activerecord-explain-analyze', '~> 0.1', require: false
 
 # OAuth
 gem 'oauth2', '~> 1.4'