Adds android version variable.

The variables in gradle.properties can be overridden on command line like: -Pversion=1.

android/app/build.gradle

         applicationId 'org.jitsi.jitsi_meet_react'
         minSdkVersion 16
         targetSdkVersion 22
-        versionCode 1
-        versionName '1.0'
+        versionCode Integer.parseInt("${version}")
+        versionName "1.0.${version}"
         ndk {
             abiFilters 'armeabi-v7a', 'x86'
         }

android/gradle.properties

 # org.gradle.parallel=true
 
 android.useDeprecatedNdk=true
+version=1