fix pushInteger() UnsatisfiedLinkError, and update gradle version.

[ichenhe]

org.keplerproject.luajava.LuaState.pushInteger() can not linked to the C function because the C function is not exist. So an exception will be threw:

java.lang.UnsatisfiedLinkError: No implementation found for void org.keplerproject.luajava.LuaState._pushInteger(org.keplerproject.luajava.CPtr, int) (tried Java_org_keplerproject_luajava_LuaState__1pushInteger and Java_org_keplerproject_luajava_LuaState__1pushInteger__Lorg_keplerproject_luajava_CPtr_2I)
                                                                    at org.keplerproject.luajava.LuaState._pushInteger(Native Method)