Updated to work with Rails 4.2+ (fix bundler issue)

I just tried updating my app to Rails 4.2.0, which was released earlier.

But when I tried to upgrade `bundler` gave me the following error:

```
Bundler could not find compatible versions for gem "activesupport":
  In Gemfile:
    rails (~> 4.2.0) ruby depends on
      activesupport (= 4.2.0) ruby

    tmdb-api (~> 0.0.8) ruby depends on
      activesupport (4.1.8)
```

I fixed this by changing the 4.1.x ActiveSupport requirement to a minimum 4.1.8 requirement.