What worked for me was to place the jniLibs folder under the "main" folder, just besides the "java" and "res" folders, for example project -> app -> src -> main -> jniLibs
I had all the libraries with the correct names and each one placed on their respective architecture subfolder, but I still had the same exception; even tried a lot of other SO answers like the accepted answer here, compiling a JAR with the .so libs, other placing of the jniLibs folder, etc.
For this project, I had to use Gradle 2.2 and Android Plugin 1.1.0 on Android Studio 1.5.1