Simplify and correct sentence parsing

Seen a couple of gems that use version numbers and what not in their gem summaries, such as [simplecov](https://github.com/colszowka/simplecov/blob/master/simplecov.gemspec#L12):

```ruby
gem.description = %Q{Code coverage for Ruby 1.9+ with a powerful configuration library and automatic merging of coverage across test suites}
```

This PR simplifies the sentence parsing and fixes the above scenario.