Set utf8 as default character encoding

[imgaojun]

Changing the default encoding to utf8 sounds good. I'm ok with that.
#270 #269

[codecov]

Codecov Report

Merging #272 into master will decrease coverage by 0.28%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #272      +/-   ##
==========================================
- Coverage   83.04%   82.75%   -0.29%     
==========================================
  Files         195      195              
  Lines       15300    15300              
==========================================
- Hits        12706    12662      -44     
- Misses       2594     2638      +44

Impacted Files	Coverage Δ
texar/torch/data/data/text_data_base.py	53.76% <ø> (ø)	⬆️
texar/torch/data/data_utils.py	43.75% <0%> (ø)	⬆️
texar/torch/data/vocabulary.py	83.69% <100%> (ø)	⬆️
texar/torch/data/data/data_iterators_utils.py	15.58% <0%> (-57.15%)	⬇️

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