Use depwarn for deprecations

[omus]

Changes deprecation warnings to use Base.depwarn instead of @warn. The primary advantage is that you can use --depwarn=no to suppress deprecations or --depwarn=error to get a full stack trace.

[codecov]

Codecov Report

Merging #704 into master will not change coverage.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##           master     #704   +/-   ##
=======================================
  Coverage   84.84%   84.84%           
=======================================
  Files          10       10           
  Lines        1847     1847           
=======================================
  Hits         1567     1567           
  Misses        280      280           

Impacted Files	Coverage Δ
src/CSV.jl	25.00% <0.00%> (ø)
src/header.jl	95.00% <100.00%> (ø)
src/utils.jl	85.50% <100.00%> (ø)
src/write.jl	83.21% <100.00%> (ø)

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[quinnj]

Thanks

[tomyun]

I believe we should have used Base.depwarn(...; force=true) here, at least for some critical changes like CSV.read(). As most users running Julia under the default --depwarn=no, there has been a slim chance seeing these deprecation warnings beforehand.

[quinnj]

Thanks for bringing this up @tomyun, I agree. I've made a PR to make a new 0.7 patch release to set force=true for the depwarns; this will allow packages on the 0.7 release to see the depwarns before updating. #782