Eliminate deprecation warnings in Julia v0.6

[wsshin]

This PR eliminates various deprecation warnings in Julia v0.6 while keeping it compatible with v0.5.

Change summary:

• Use dot syntax for min, max, sqrt, etc
• sumabs2 → sum(abs2, ...)
• abstract TypeName → @compat abstract type TypeName end
• The v0.6 inner constructor syntax cannot be used even with the Compat package (see here), so use the workaround suggested here.

[codecov-io]

Codecov Report

Merging #2 into master will decrease coverage by 1.56%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #2      +/-   ##
==========================================
- Coverage   85.04%   83.48%   -1.57%     
==========================================
  Files           6        6              
  Lines         107      109       +2     
==========================================
  Hits           91       91              
- Misses         16       18       +2

Impacted Files	Coverage Δ
src/GeometryPrimitives.jl	100% <ø> (ø)	⬆️
src/sphere.jl	100% <100%> (ø)	⬆️
src/kdtree.jl	74.5% <100%> (-3.5%)	⬇️
src/box.jl	93.75% <100%> (ø)	⬆️
src/ellipsoid.jl	100% <100%> (ø)	⬆️
src/cylinder.jl	84% <100%> (ø)	⬆️

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[stevengj]

Thanks!