Remove activerecord-session_store gem

We use the Redis-backed session store from `redis-store` so this is
unnecessary.

Gemfile

@@ -334,7 +334,6 @@ gem 'email_reply_parser', '~> 0.5.8'
 gem 'ruby-prof', '~> 0.16.2'
 ## CI
-gem 'activerecord-session_store', '~> 1.0.0'
 gem 'nested_form', '~> 0.3.2'
 # OAuth

Gemfile.lock

@@ -32,12 +32,6 @@ GEM
       activemodel (= 4.2.7.1)
       activesupport (= 4.2.7.1)
       arel (~> 6.0)
-    activerecord-session_store (1.0.0)
-      actionpack (>= 4.0, < 5.1)
-      activerecord (>= 4.0, < 5.1)
-      multi_json (~> 1.11, >= 1.11.2)
-      rack (>= 1.5.2, < 3)
-      railties (>= 4.0, < 5.1)
     activerecord_sane_schema_dumper (0.2)
       rails (>= 4, < 5)
     activesupport (4.2.7.1)
@@ -809,7 +803,6 @@ PLATFORMS
 DEPENDENCIES
   RedCloth (~> 4.3.2)
   ace-rails-ap (~> 4.1.0)
-  activerecord-session_store (~> 1.0.0)
   activerecord_sane_schema_dumper (= 0.2)
   acts-as-taggable-on (~> 4.0)
   addressable (~> 2.3.8)