Stop making Tracker loading depend on version

[nickrobinson251]

• No reliable way to identify the version
• More useful to assume a compatible Tracker
• see Make Tracker an optional dependency #54 (comment) (Thanks @KristofferC for noticing this)

[oxinabox]

Possibly solution in JuliaPackaging/Requires.jl#3 (comment)
but I would rather push that into Requires than do it here.
So lets do this

[codecov]

Codecov Report

Merging #57 into master will increase coverage by 0.13%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #57      +/-   ##
==========================================
+ Coverage   85.97%   86.11%   +0.13%     
==========================================
  Files           8        8              
  Lines         221      216       -5     
==========================================
- Hits          190      186       -4     
+ Misses         31       30       -1

Impacted Files	Coverage Δ
src/NamedDims.jl	100% <100%> (+14.28%)	⬆️

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