add SoLoader's proaguard rule

MapboxGLAndroidSDK/proguard-rules.pro

@@ -38,4 +38,10 @@
 # while we don't include this dependency directly
 # a large amount of users combine it with our SDK
 # we aren't able to provide a proguard config in that project (jar vs aar)
--dontwarn com.sun.xml.internal.ws.spi.db.*
+-dontwarn com.sun.xml.internal.ws.spi.db.*
+
+# config for SoLoader https://github.com/mapbox/mapbox-gl-native-android/issues/60,
+# https://github.com/facebook/SoLoader/blob/master/java/com/facebook/soloader/soloader.pro
+-keep class com.facebook.soloader.SysUtil$LollipopSysdeps {
+    public <methods>;
+}