Resolve signature issue

[willtebbutt]

Addresses #39

I'm not sure if it resolves it correctly, so it would be helpful if someone could check that this is the correct behaviour.

[codecov]

Codecov Report

Merging #40 (a87229c) into master (124baaa) will decrease coverage by 1.29%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #40      +/-   ##
==========================================
- Coverage   97.39%   96.10%   -1.29%     
==========================================
  Files           5        5              
  Lines         230      231       +1     
==========================================
- Hits          224      222       -2     
- Misses          6        9       +3     

Files Changed	Coverage Δ
src/type_utils.jl	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[willtebbutt]

I've update LTS, and removed CI on 1.5 (because Invenia no longer uses 1.5)

[willtebbutt]

Some lines have been uncovered via the switch to the 1.6 LTS, hence the small drop in coverage. Not sure what we want to do

[oxinabox]

LGTM, merge whenever you want