Typo: Change tl.logging --> logging in nlp.py (#718)
Typo: Change tl.logging --> logging in nlp.py (#718)

Change to match line 20 and resolve _undefined names_ which have the potential to raise NameError at runtime.

flake8 testing of https://github.com/tensorlayer/tensorlayer on Python 3.6.3

$ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./tensorlayer/nlp.py:283:13: F821 undefined name 'tl'
            tl.logging.fatal("Vocab file %s not found." % vocab_file)
            ^
./tensorlayer/nlp.py:284:9: F821 undefined name 'tl'
        tl.logging.info("Initializing vocabulary from file: %s" % vocab_file)
        ^
./tensorlayer/nlp.py:1102:9: F821 undefined name 'tl'
        tl.logging.info("Unable to fetch multi-bleu.perl script, using local.")
        ^
./tensorlayer/nlp.py:1130:17: F821 undefined name 'tl'
                tl.logging.warning("multi-bleu.perl script returned non-zero exit code")
                ^
./tensorlayer/nlp.py:1131:17: F821 undefined name 'tl'
                tl.logging.warning(error.output)
                ^
./tests/test_yapf_format.py:20:20: F821 undefined name 'unicode'
            return unicode(f.read(), 'utf-8')
                   ^
6     F821 undefined name 'tl'
6
```
zsdonghao/tensorlayer,zsdonghao/tensorlayer