Drop 0.5 support

[iblislin]

TimeSeries.jl 0.11.1 released.
Time to drop 0.5 support.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6ddfdaa). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #103   +/-   ##
=========================================
  Coverage          ?   93.71%           
=========================================
  Files             ?        7           
  Lines             ?      207           
  Branches          ?        0           
=========================================
  Hits              ?      194           
  Misses            ?       13           
  Partials          ?        0

Impacted Files	Coverage Δ
src/momentum.jl	100% <100%> (ø)

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