Removes targetSdkVersion="19" restriction

This leaves it up for the project that is including this library to define what the ``targetSdkVersion`` will be.

```
[/Users/felipecsl/myproject/main/AndroidManifest.xml:5, /Users/felipecsl/myproject/build/exploded-aar/com.sothree.slidinguppanel/library/1.0.1/AndroidManifest.xml:3] Main manifest has <uses-sdk android:targetSdkVersion='18'> but library uses targetSdkVersion='19'
:myproject:processDefaultFlavorDebugManifest FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':myproject:processDefaultFlavorDebugManifest'.
> Manifest merging failed. See console for more info.

* Try:
Run with --stacktrace option to get the stack trace. Run with --info or --debug option to get more log output.
```