Towards Julia 0.6 compatibility

[tkoolen]

Fix a bunch of issues on Julia 0.6. Not quite there yet; there are some segfaults and illegal instruction errors while running tests, on the same line as referenced in JuliaLang/julia#21216, so I'm hoping that once that issue gets fixed, my problems go away as well.

[codecov]

Codecov Report

Merging #197 into master will not change coverage.
The diff coverage is 95%.

@@           Coverage Diff           @@
##           master     #197   +/-   ##
=======================================
  Coverage   94.91%   94.91%           
=======================================
  Files          18       18           
  Lines        2046     2046           
=======================================
  Hits         1942     1942           
  Misses        104      104

Impacted Files	Coverage Δ
src/frames.jl	90.9% <100%> (ø)	⬆️
src/joint.jl	93.75% <100%> (ø)	⬆️
src/third_party_addendum.jl	95.45% <100%> (ø)	⬆️
src/rigid_body.jl	90.9% <100%> (ø)	⬆️
src/graphs.jl	90.85% <100%> (ø)	⬆️
src/contact.jl	87.95% <100%> (ø)	⬆️
src/spatial.jl	93.26% <100%> (ø)	⬆️
src/parse_urdf.jl	93.67% <100%> (ø)	⬆️
src/util.jl	98.95% <80%> (ø)	⬆️

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