Drop support for Julia 0.7

[DilumAluthge]

I don't think we need to keep supporting Julia 0.7 at this point.

As a point of reference, here is the current Julia [compat] entry for Coverage.jl:

[compat]
julia = "1"

[codecov]

Codecov Report

Merging #65 (90584fe) into master (8d5c61d) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master       #65   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          240       240           
=========================================
  Hits           240       240           

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