Add exponential backoff

[rohitpaulk]

This PR adds exponential backoff when requests are retried.

I picked the default backoff parameter values from https://github.com/segmentio/analytics-go and https://github.com/segmentio/backo-go.

[codecov-io]

Codecov Report

Merging #134 into master will decrease coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
- Coverage   98.85%   98.74%   -0.11%     
==========================================
  Files          12       14       +2     
  Lines         612      799     +187     
==========================================
+ Hits          605      789     +184     
- Misses          7       10       +3

Impacted Files	Coverage Δ
lib/segment/analytics/request.rb	100% <100%> (ø)	⬆️
lib/segment/analytics/defaults.rb	100% <100%> (ø)	⬆️
lib/segment/analytics/backoff_policy.rb	100% <100%> (ø)
spec/segment/analytics/request_spec.rb	100% <100%> (ø)	⬆️
spec/segment/analytics/client_spec.rb	98.1% <0%> (ø)

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[f2prateek]

was this previously unused?

[rohitpaulk]

Yes. It was being assigned to a variable, but the actual headers used were hardcoded.

(The diff for this individual commit includes all the relevant files).