allow railties (=4.0)

Hello @ispyropoulos.

I have the following issue:
```
$ bundle
Fetching gem metadata from https://rubygems.org/.........
Fetching additional metadata from https://rubygems.org/..
Resolving dependencies...
Bundler could not find compatible versions for gem "railties":
  In snapshot (Gemfile.lock):
    railties (4.0.10)

  In Gemfile:
    intl-tel-input-rails (~> 3.6.0) ruby depends on
      railties (~> 3.1) ruby

Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.

$ bundle update
Updating git://github.com/plataformatec/simple_form.git
Updating git://github.com/carrierwaveuploader/carrierwave.git
Fetching gem metadata from https://rubygems.org/.........
Fetching additional metadata from https://rubygems.org/..
Resolving dependencies...
Bundler could not find compatible versions for gem "railties":
  In Gemfile:
    intl-tel-input-rails (~> 3.6.0) ruby depends on
      railties (~> 3.1) ruby

    rails (= 4.0.10) ruby depends on
      railties (4.0.10)
$
```

According to [this](http://bundler.io/v1.7/gemfile.html), ```~> 3.1``` is identical to ```>= 3.1 and < 3.2```
is there a special need to use only railties 3.1.* ?

As I use rails 4.0.10, it needs railties 4.0.10.