Simplify TAI-UTC calculation for pre-leapsecond dates

[helgee]

Follow-up to #31

@prakharcode your troubles with calculating the drift correctly were based on a misunderstanding of mine. The reference epoch for the drift can also be before the current time, see here: ftp://maia.usno.navy.mil/ser7/tai-utc.dat

[coveralls]

Pull Request Test Coverage Report for Build 191

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 87.371%

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Totals
Change from base Build 189:	-0.2%
Covered Lines:	339
Relevant Lines:	388

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💛 - Coveralls

[codecov-io]

Codecov Report

Merging #33 into master will decrease coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   87.53%   87.37%   -0.17%     
==========================================
  Files           7        7              
  Lines         393      388       -5     
==========================================
- Hits          344      339       -5     
  Misses         49       49

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

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