Disable attribution tracking by default

[f2prateek]

Closes #532

[codecov-io]

Codecov Report

Merging #534 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #534   +/-   ##
=======================================
  Coverage   82.97%   82.97%           
=======================================
  Files          30       30           
  Lines        2426     2426           
  Branches      275      275           
=======================================
  Hits         2013     2013           
  Misses        330      330           
  Partials       83       83

Impacted Files	Coverage Δ
...src/main/java/com/segment/analytics/Analytics.java	75.34% <100%> (ø)	⬆️

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