Upgrade Android support libraries to the latest version

While at it, extract the common version to a constant.

Change-Id: Ifac7ba0ee16d6978f1432faea9206891ff65ed1a

app/build.gradle

diff --git a/app/build.gradle b/app/build.gradle
index f830bd3..338f95c 100644
--- a/app/build.gradle
+++ b/app/build.gradle
@@ -23,10 +23,14 @@
 
 dependencies {
     testCompile 'junit:junit:4.12'
+
     compile 'com.github.uwolfer:gerrit-rest-java-client:dd4837b'
+
     compile 'com.google.guava:guava:19.0'
     compile 'com.google.code.gson:gson:2.6.2'
-    compile 'com.android.support:appcompat-v7:23.1.1'
-    compile 'com.android.support:design:23.1.1'
-    compile 'com.android.support:support-v4:23.1.1'
+
+    final ANDROID_SUPPORT_VERSION = '25.0.1'
+    compile "com.android.support:appcompat-v7:${ANDROID_SUPPORT_VERSION}"
+    compile "com.android.support:design:${ANDROID_SUPPORT_VERSION}"
+    compile "com.android.support:support-v4:${ANDROID_SUPPORT_VERSION}"
 }