Pinning a version of click to fix travis

[michellethomas]

Click recently came out with a new version where "Subcommands that are named by the function now automatically have the underscore replaced with a dash. If you register a function named my_command it becomes my-command in the command line interface." This breaks tests as it affects some cli commands superset load_examples is now superset load-examples.

Pinning to the previous version of click before this change to fix travis.
@williaster @john-bodley @kristw

[codecov-io]

Codecov Report

Merging #5975 into master will increase coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5975      +/-   ##
==========================================
+ Coverage   63.52%   63.56%   +0.04%     
==========================================
  Files         393      393              
  Lines       23667    23667              
  Branches     2638     2638              
==========================================
+ Hits        15034    15044      +10     
+ Misses       8620     8610      -10     
  Partials       13       13

Impacted Files	Coverage Δ
superset/connectors/sqla/models.py	81.13% <0%> (+0.75%)	⬆️
superset/db_engine_specs.py	55.7% <0%> (+0.87%)	⬆️

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[kristw]

LGTM

[mistercrunch]

Sounds like that means that installing using pip install superset won't work. We should fix setup.py as well.