update gemspec dependencies

## motivation

I want to relax dependencies so we don't have problems updating the gem later.

I would also like to make the following changes, but need guidance.

#### 1. should only include the files necessary for runtime

The current `git ls-files` strategy pulls too many files into gem; files that are not required at runtime.

```
+  s.files         = Dir.glob('{bin,lib}/**/*') + Dir.glob('scripts/*.js') + ['scripts/udidetect', 'LICENSE']
```

#### 2. update your email address

```
+  s.email       = ['karl.krukow@xamarin.com']
```

**IMPORTANT** Neither of the changes above are in the changeset!

## todo

- [ ] @krukow needs review