Set scalatest as testCompile dependency

Gradle now generates correct eclipse dependencies
Actually, it's an Android special one. Using these libraries in android development is much like using JDK runtime. It's better that we don't add these dependencies to rxjava-android. gradle android plugin will set up the environment according to the settings, such as:
```
apply plugin: 'android'

android {
    buildToolsVersion "19.0.3"
    compileSdkVersion 19
}
```