Updated build.gradle, gradle-wrapper,and WeatherService

updated build.gradle
 -bumped compileSdkVersion to 21
 -bumped buildToolsVersion to 21.1.2
 -bumped targetSdkVersion to 21
 -modified "runProguard false" to "minifyEnabled false" to reflect api change

updated android-gradle plugin
 -bumped to 1.0.0

updated gradle-wrapper.properties to use latest gradle version 2.2.1

updated WeatherService to reflect api changes
-"return Observable.from(this);" to "return Observable.just(this);"