Allow ActiveSupport versions above 3.2

I am attempting to use this gem in a application Rails 4.1 application. I get this from bundler:

```
Resolving dependencies...
Bundler could not find compatible versions for gem "activesupport":
  In snapshot (Gemfile.lock):
    activesupport (4.1.4)

  In Gemfile:
    rxsd (>= 0) ruby depends on
      activesupport (~> 3.2.11) ruby

Running `bundle update` will rebuild your snapshot from scratch, using only
the gems in your Gemfile, which may resolve the conflict.
```

ActiveSupport is used in https://github.com/movitto/RXSD/blob/454efb1f20d095d54186add9c90c25810ecebf12/lib/rxsd/common.rb#L7 

ActiveSupport 4.1.4 might work just fine:
```
irb(main):001:0> require 'active_support/inflector'
=> true
irb(main):002:0> ActiveSupport::Inflector
=> ActiveSupport::Inflector
irb(main):003:0> require 'active_support/version'
=> true
irb(main):004:0> ActiveSupport::VERSION::STRING
=> "4.1.4"
```