Update README reference to MapD to say OmniSci

[randyzwitch]

Update MapD references to OmniSci

[cpcloud]

I'm happy to rename other things as well (ideally preserving backwards compatibility). Is there anything else you want to rename?

[randyzwitch]

I just targeted the README since it was pointing to the wrong website link. How things change inside the codebase is probably a much larger effort, and probably should coordinate with @xmnlab.

[codecov]

Codecov Report

Merging #1749 into master will decrease coverage by 1.9%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1749      +/-   ##
==========================================
- Coverage   87.33%   85.42%   -1.91%     
==========================================
  Files          80       80              
  Lines       15037    15037              
  Branches     1923     1923              
==========================================
- Hits        13132    12846     -286     
- Misses       1558     1846     +288     
+ Partials      347      345       -2

Impacted Files	Coverage Δ
ibis/bigquery/client.py	41.1% <0%> (-53.39%)	⬇️
ibis/bigquery/compiler.py	58.67% <0%> (-38.75%)	⬇️
ibis/bigquery/udf/api.py	80.48% <0%> (-14.64%)	⬇️
ibis/bigquery/api.py	63.33% <0%> (-10.01%)	⬇️
ibis/impala/compiler.py	91.04% <0%> (-5.75%)	⬇️
ibis/pandas/client.py	86.54% <0%> (-3.51%)	⬇️
ibis/expr/schema.py	89.77% <0%> (-1.14%)	⬇️
ibis/expr/window.py	86.77% <0%> (-0.83%)	⬇️
ibis/expr/api.py	93.55% <0%> (-0.4%)	⬇️
ibis/sql/compiler.py	93.7% <0%> (-0.39%)	⬇️
... and 2 more