DOC: remove references to oauth2client

[tswast]

Closes #59.

[codecov-io]

Codecov Report

Merging #61 into master will decrease coverage by 45.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   73.44%   28.24%   -45.2%     
==========================================
  Files           4        4              
  Lines        1540     1540              
==========================================
- Hits         1131      435     -696     
- Misses        409     1105     +696

Impacted Files	Coverage Δ
pandas_gbq/gbq.py	19.35% <0%> (-55.98%)	⬇️
pandas_gbq/tests/test_gbq.py	27.94% <0%> (-54.94%)	⬇️

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[parthea]

Thanks @tswast !