CI/Travis: Fix tagged builds

shell/android-studio/reicast/build.gradle

@@ -1,11 +1,20 @@
 apply plugin: 'com.android.application'
 apply plugin: 'com.github.triplet.play'
 
-def getVersionCode = { ->
+def getBuildId = { ->
     def build_id = System.getenv("TRAVIS_JOB_ID") ?: "8"
     return Integer.parseInt( build_id )
 }
 
+def getVersionHash = { ->
+    def stdout = new ByteArrayOutputStream()
+    exec {
+        commandLine 'git', 'rev-parse', '--short', 'HEAD'
+        standardOutput = stdout
+    }
+    return stdout.toString().trim()
+}
+
 def getVersionName = { ->
     def stdout = new ByteArrayOutputStream()
     exec {
@@ -23,7 +32,7 @@ android {
         applicationId "com.reicast.emulator"
         minSdkVersion 16
         targetSdkVersion 25
-        versionCode getVersionCode()
+        versionCode getBuildId()
         versionName getVersionName()
         vectorDrawables.useSupportLibrary = true
 
@@ -72,9 +81,7 @@ android {
 afterEvaluate {
     android.applicationVariants.all {  v ->
         if (v.buildType.name == "release") {
-            def hashtag = getVersionName()
-            def start = hashtag.lastIndexOf("-")
-            hashtag = hashtag.substring(start + 2, start + 9)
+            def hashtag = getVersionHash()
             v.outputs[0].outputFileName = "reicast-android-" + hashtag + ".apk"
         }
     }