sentinel-datasource-xxx not support jdk1.8.0_20

for example
when you use sentinel-datasource-nacos or sentinel-datasource-apollo
if your jdk is 1.8.0_20, the application start with error "Failed to bind properties under 'spring.cloud.sentinel.datasource.ds.nacos.rule-type'" to org.springframework.cloud.alibaba.sentinel.datasource.RuleType
but other 1.8version of jdk is avaliable
