Add tests and upgrade to more recent Julia versions

[timholy]

This adds some tests and upgrades the package to reasonably modern Julia versions

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@a16aece). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master       #2   +/-   ##
=========================================
  Coverage          ?   93.33%           
=========================================
  Files             ?        1           
  Lines             ?       90           
  Branches          ?        0           
=========================================
  Hits              ?       84           
  Misses            ?        6           
  Partials          ?        0

Impacted Files	Coverage Δ
src/mzXML.jl	93.33% <100%> (ø)

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