Rename carrier identifiers

[danh91]

Rename carrier identifier keys for clarity

• renaming: carrier -> carrier_name
• renaming: carrier_name -> carrier_id
• update tests
• bump version to 2020.6.1

[codecov-commenter]

Codecov Report

Merging #64 into master will increase coverage by 0.00%.
The diff coverage is 96.55%.

@@           Coverage Diff           @@
##           master      #64   +/-   ##
=======================================
  Coverage   94.31%   94.31%           
=======================================
  Files         116      116           
  Lines        4783     4786    +3     
  Branches      232      232           
=======================================
+ Hits         4511     4514    +3     
  Misses        224      224           
  Partials       48       48           

Impacted Files	Coverage Δ
.../canadapost/purplship/carriers/canadapost/error.py	100.00% <ø> (ø)
...canadapost/purplship/carriers/canadapost/rating.py	94.11% <ø> (ø)
.../carriers/canadapost/shipment/contract_shipment.py	94.87% <ø> (ø)
...riers/canadapost/shipment/non_contract_shipment.py	72.97% <ø> (ø)
.../canadapost/purplship/carriers/canadapost/track.py	100.00% <ø> (ø)
...tensions/dhl/purplship/carriers/dhl/book_pickup.py	100.00% <ø> (ø)
extensions/dhl/purplship/carriers/dhl/dct_quote.py	86.27% <ø> (ø)
extensions/dhl/purplship/carriers/dhl/error.py	100.00% <ø> (ø)
...nsions/dhl/purplship/carriers/dhl/modify_pickup.py	100.00% <ø> (ø)
extensions/dhl/purplship/carriers/dhl/ship_val.py	90.47% <ø> (ø)
... and 40 more

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