Fix alias in multi_logger

[motoroller95]

When I use MultiLogger#log I expect that all loggers will receive my message. Actually Logger#log is alias to Logger#add and because ruby alias is not apply to child classes, when I write MultiLogger.new(...).log loggers will no receive message, but Logger#log will

@dwbutler Hi! pls review this

[codecov-commenter]

Codecov Report

Merging #161 into master will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
- Coverage   92.68%   92.64%   -0.04%     
==========================================
  Files          64       64              
  Lines        1668     1673       +5     
==========================================
+ Hits         1546     1550       +4     
- Misses        122      123       +1     

Impacted Files	Coverage Δ
lib/logstash-logger/multi_logger.rb	75.34% <100.00%> (+3.12%)	⬆️
spec/multi_logger_spec.rb	100.00% <100.00%> (ø)
lib/logstash-logger/device/kinesis.rb	88.88% <0.00%> (-5.56%)	⬇️
lib/logstash-logger/device/firehose.rb	88.88% <0.00%> (-5.56%)	⬇️
lib/logstash-logger/device/tcp.rb	94.11% <0.00%> (-1.97%)	⬇️

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