Use gradle constants and update GSON

app/build.gradle

@@ -12,26 +12,24 @@ dependencies {
     compile 'ch.acra:acra:4.7.0'
     compile 'org.mediawiki:api:1.3'
     compile 'commons-codec:commons-codec:1.10'
-    compile 'com.android.support:support-v4:25.2.0'
-    compile 'com.android.support:appcompat-v7:25.2.0'
-    compile 'com.android.support:design:25.2.0'
+    compile "com.android.support:support-v4:${project.supportLibVersion}"
+    compile "com.android.support:appcompat-v7:${project.supportLibVersion}"
+    compile "com.android.support:design:${project.supportLibVersion}"
+    compile 'com.google.code.gson:gson:2.7'
 
     testCompile 'junit:junit:4.12'
-
-    //noinspection GradleDependency - old version has required feature
-    compile 'com.google.code.gson:gson:1.4'
 }
 
 android {
-    compileSdkVersion 25
-    buildToolsVersion '25.0.1'
+    compileSdkVersion project.compileSdkVersion
+    buildToolsVersion project.buildToolsVersion
 
-    useLibrary  'org.apache.http.legacy'
+    useLibrary 'org.apache.http.legacy'
 
     defaultConfig {
-        applicationId "fr.free.nrw.commons"
-        minSdkVersion 15
-        targetSdkVersion 25
+        applicationId 'fr.free.nrw.commons'
+        minSdkVersion project.minSdkVersion
+        targetSdkVersion project.targetSdkVersion
     }
 
     buildTypes {

build.gradle

@@ -5,7 +5,7 @@ buildscript {
         mavenCentral()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:2.3.0'
+        classpath "com.android.tools.build:gradle:${project.gradleVersion}"
     }
 }
 

gradle.properties

@@ -0,0 +1,9 @@
+gradleVersion = 2.3.0
+
+supportLibVersion = 25.2.0
+
+compileSdkVersion = android-25
+buildToolsVersion = 25.0.1
+
+minSdkVersion = 15
+targetSdkVersion = 25