TST: Use private key for unicode tests

[tswast]

Fixes tests added in #108 for Travis.

/cc @maxim-lian

[tswast]

Successful build at https://travis-ci.org/tswast/pandas-gbq/builds/330613455

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@3b112bf). Click here to learn what that means.
The diff coverage is 0%.

@@            Coverage Diff            @@
##             master     #111   +/-   ##
=========================================
  Coverage          ?   28.56%           
=========================================
  Files             ?        4           
  Lines             ?     1537           
  Branches          ?        0           
=========================================
  Hits              ?      439           
  Misses            ?     1098           
  Partials          ?        0

Impacted Files	Coverage Δ
pandas_gbq/tests/test_gbq.py	27.37% <0%> (ø)

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[max-sixty]

Thanks a lot @tswast