2. fix some issue

1. pmd -> 6.15.0
2. idea min version 2016.1
3. jdk min version 1.8

This fixes https://github.com/alibaba/p3c/issues/471

Previously, the following code can't pass EqualsAvoidNullRule:

public class Test {
  private static final String VERSION = System.getProperty("v");
  public boolean isJava6(){
    return "1.6".equals(VERSION);
  }
}

This PR fixes this issue by checking if the caller is a literal.

 [Recommended] The total number of lines for a method should not be more than 80
 [Recommended] Avoid using the negation operator '!'
 [Mandatory] When doing date formatting, "y" should be written in lowercase for "year" in a pattern statement

 and some bug fix

https://github.com/alibaba/p3c/issues/107