Update gradle, remove HTML build script

Jetty was deprecated and should be replaced with Gretty, but
it didn't work for HTML anyway (#56) so instead I have decided
to remove it entirely for now (it can always be added back in
a future if someone is willing to do so).

android/AndroidManifest.xml

@@ -4,7 +4,7 @@
     android:versionCode="1"
     android:versionName="1.0" >
 
-    <uses-sdk android:minSdkVersion="8" android:targetSdkVersion="25" />
+    <uses-sdk android:targetSdkVersion="25" />
 
     <application
         android:allowBackup="true"

android/build.gradle

@@ -12,7 +12,7 @@ android {
             jniLibs.srcDirs = ['libs']
         }
 
-        instrumentTest.setRoot('tests')
+        androidTest.setRoot('tests')
     }
     packagingOptions {
         exclude 'META-INF/robovm/ios/robovm.xml'