Update gradle files

gradle/wrapper/gradle-wrapper.properties

@@ -2,4 +2,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-2.4-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-2.6-all.zip

traccar-client/build.gradle

@@ -23,7 +23,6 @@ android {
     buildTypes {
         release {
             minifyEnabled false
-            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
             signingConfig signingConfigs.release
         }
     }