use a frozen copy of Rails.root in AppNotifications and LogInterceptor

i was playing with https://github.com/schneems/derailed_benchmarks while debugging a memory leak in an application and i saw that meta_request was allocating a lot of memory:

    $ bundle exec rake -f perf.rake perf:mem RAILS_ENV=development
    Booting: development
    Running 1 times
    Total allocated 155114
    Total retained 1160

    allocated memory by gem
    -----------------------------------
    meta_request-0.3.4 x 17136862
    activesupport-4.1.8 x 700213
    haml-4.0.6 x 346715
    actionview-4.1.8 x 328915
    ruby-2.1.5/lib x 327681```

the specific culprits were:

    allocated memory by location
    -----------------------------------
    gems/meta_request-0.3.4/lib/meta_request/app_notifications.rb:11 x 13325502
    gems/meta_request-0.3.4/lib/meta_request/log_interceptor.rb:39 x 3758621

both of which involve converting ```Rails.root``` from a ```Pathname``` into a ```String```. 

this change adds a ```MetaRequest.rails_root``` method which caches a frozen copy of ```Rails.root.to_s``` and drops the total memory allocation of a rails boot to 40% of the previous value:

    Booting: development
    Running 1 times
    Total allocated 67646
    Total retained 1217

    allocated memory by gem
    -----------------------------------
    meta_request/lib x 6846874
    activesupport-4.1.8 x 700143
    haml-4.0.6 x 346420
    actionview-4.1.8 x 328915
    ruby-2.1.5/lib x 327681

    allocated memory by location
    -----------------------------------
    git/rails_panel/meta_request/lib/meta_request/log_interceptor.rb:39 x 3663501
    git/rails_panel/meta_request/lib/meta_request/app_notifications.rb:11 x 3130480