remove log4j from dependencies.

[oujinliang]

I found this java client depends on log4j:

    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-log4j12</artifactId>
      <version>1.7.2</version>
    </dependency>

it's not good to bind a slf4j implementation like log4j in the library, many applications use other slf4j bindings like logback or log4j2.

[foreverneverer]

Thanks，we will remove it if it's reasonable and feasible

[foreverneverer]

in #70, we are changing to log4j2. welcome to review it.