Replacing log4j dependency with slf4j

[mccartney]

Two major changes:

• dropping the dependency on log4j. There's no meta usage here, it was just for logging actions from this library. slf4j gives more flexibility for downstream users
• upgrading to Java 8

[codecov-commenter]

Codecov Report

Merging #23 (a04e9f2) into master (72bf6dc) will decrease coverage by 0.71%. The diff coverage is 85.71%.

:exclamation: Current head a04e9f2 differs from pull request most recent head 84eb411. Consider uploading reports for the commit 84eb411 to get more accurate results

@@             Coverage Diff              @@
##             master      #23      +/-   ##
============================================
- Coverage     62.22%   61.51%   -0.72%     
+ Complexity       66       65       -1     
============================================
  Files             9        9              
  Lines           323      317       -6     
  Branches         28       27       -1     
============================================
- Hits            201      195       -6     
  Misses          108      108              
  Partials         14       14              

Impacted Files	Coverage Δ
...umologic/http/sender/HttpProxySettingsCreator.java	0.00% <0.00%> (ø)
...sumologic/http/aggregation/BufferFlushingTask.java	91.66% <100.00%> (-1.44%)	:arrow_down:
.../sumologic/http/aggregation/SumoBufferFlusher.java	92.30% <100.00%> (ø)
...m/sumologic/http/queue/BufferWithFifoEviction.java	80.76% <100.00%> (ø)
.../sumologic/http/sender/SumoBufferFlushingTask.java	90.47% <100.00%> (ø)
...java/com/sumologic/http/sender/SumoHttpSender.java	82.65% <100.00%> (-0.18%)	:arrow_down:

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[mccartney]

Seemingly this had nothing to do with this PR, but I've just spent a few hours debugging it and discovered bug reported in #24.