Change target of gradle build to Android 5.1.

briar-android/build.gradle

@@ -9,8 +9,8 @@ dependencies {
 }
 
 android {
-    compileSdkVersion 20
-    buildToolsVersion "20.0.0"
+    compileSdkVersion 22
+    buildToolsVersion "22.0.1"
 
     sourceSets {
         main {