deprecate dependency_links unless --use-dependency-links specified

[jbraswell]

Pip removed support for dependency_links in 19.0 (pypa/pip#6060). This PR modifies the sync command to write dependencies to install_requires using PEP 508 URLs, unless --use-dependency-links is specified.

[codecov-commenter]

Codecov Report

Merging #62 (3b4c469) into master (2b66c23) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
+ Coverage   94.63%   94.70%   +0.07%     
==========================================
  Files          10       10              
  Lines         745      756      +11     
==========================================
+ Hits          705      716      +11     
  Misses         40       40              

Impacted Files	Coverage Δ
pipenv_setup/lockfile_parser.py	94.00% <100.00%> (+0.66%)	⬆️
pipenv_setup/main.py	94.85% <100.00%> (+0.03%)	⬆️
pipenv_setup/pipfile_parser.py	89.39% <100.00%> (+0.86%)	⬆️
pipenv_setup/setup_parser.py	88.57% <100.00%> (+1.42%)	⬆️
pipenv_setup/setup_updater.py	96.82% <0.00%> (-0.80%)	⬇️

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[jbraswell]

Closed in favor of #64