Step android gradle plugin version to 1.5.0

Gradle 2.8 require Android Gradle plugin version
1.2.0 or newer.
Upgrade plugin version to 1.5.0.
Add multiDex support to prevent
DexIndexOverflowException.

build.gradle

@@ -6,7 +6,7 @@ buildscript {
         mavenCentral()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:1.0.0'
+        classpath 'com.android.tools.build:gradle:1.5.0'
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files
     }

mobile/build.gradle

@@ -8,6 +8,7 @@ android {
         applicationId "org.openhab.habdroid"
         minSdkVersion 15
         targetSdkVersion 21
+        multiDexEnabled true
     }
     buildTypes {
         release {
@@ -38,4 +39,5 @@ dependencies {
     compile 'com.loopj:android-smart-image-view:1.0.0'
     compile 'com.google.code.gson:gson:2.3.1'
     compile 'com.github.shell-software:fab:1.1.0'
+    compile 'com.android.support:multidex:'
 }