fix ambiguities on 1.8 & nightly

[Roger-luo]

OK this should fix the nightly ambiguity - can we have a new patch release after this gets merged?

[codecov-commenter]

Codecov Report

Merging #116 (d0b9bd3) into master (d2edc82) will decrease coverage by 0.19%.
The diff coverage is 0.00%.

❗ Current head d0b9bd3 differs from pull request most recent head 679fb6b. Consider uploading reports for the commit 679fb6b to get more accurate results

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
- Coverage   95.73%   95.54%   -0.20%     
==========================================
  Files          12       13       +1     
  Lines         727      740      +13     
==========================================
+ Hits          696      707      +11     
- Misses         31       33       +2     

Impacted Files	Coverage Δ
src/Measurements.jl	87.87% <ø> (ø)
src/compat.1.8.jl	0.00% <0.00%> (ø)
src/utils.jl	100.00% <0.00%> (ø)
src/derivatives-type.jl	100.00% <0.00%> (ø)
src/math.jl	98.95% <0.00%> (+0.01%)	⬆️
src/conversions.jl	100.00% <0.00%> (+8.33%)	⬆️

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