Allow setting versionCode in parameter (#276)

Allow easier debug versionCode change

app/build.gradle

@@ -38,7 +38,7 @@ android {
         minSdkVersion 16
         targetSdkVersion 23
         testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
-        versionCode 6
+        versionCode project.getProperties().get("versionCode")?.toInteger() ?: 6
         versionName "0.2.0"
 
         buildConfigField "String", "COMMIT_COUNT", "\"${getCommitCount()}\""