[Security] Bump nltk from 3.4 to 3.5 in /travis

[dependabot-preview]

Bumps nltk from 3.4 to 3.5. This update includes a security fix.

Vulnerabilities fixed

Sourced from The GitHub Security Advisory Database.

Moderate severity vulnerability that affects nltk NLTK Downloader before 3.4.5 is vulnerable to a directory traversal, allowing attackers to write arbitrary files via a ../ (dot dot slash) in an NLTK package (ZIP archive) that is mishandled during extraction.

Affected versions: < 3.4.5

Changelog

Sourced from nltk's changelog.

Version 3.5 2020-04-13

• add support for Python 3.8
• drop support for Python 2
• create NLTK's own Tokenizer class distinct from the Treebank reference tokeniser
• update Vader sentiment analyser
• fix JSON serialization of some PoS taggers
• minor improvements in grammar.CFG, Vader, pl196x corpus reader, StringTokenizer
• change implementation = for FreqDist so they are partial orders
• make FreqDist iterable
• correctly handle Penn Treebank trees with a unlabeled branching top node.

Thanks to the following contributors to 3.5: Nicolas Darr, Gerhard Kremer, Liling Tan, Christopher Hench, Alexandre Dias, Hervé Nicol, Pierpaolo Pantone, Bonifacio de Oliveira, Maciej Gawinecki, BLKSerene, hoefling, alvations, pyfisch, srhrshr

Version 3.4.5 2019-08-20

• Fixed security bug in downloader: Zip slip vulnerability - for the unlikely situation where a user configures their downloader to use a compromised server https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-14751)

Thanks to the following contributors to 3.4.5: Mike Salvatore

Version 3.4.4 2019-07-04

• fix bug in plot function (probability.py)
• add improved PanLex Swadesh corpus reader

Thanks to the following contributors to 3.4.4: Devashish Lal, Liling Tan

Version 3.4.3 2019-06-07

• add Text.generate()
• add QuadgramAssocMeasures
• add SSP to tokenizers
• return confidence of best tag from AveragedPerceptron
• make plot methods return Axes objects
• don't require list arguments to PositiveNaiveBayesClassifier.train
• fix Tree classes to work with native Python copy library
• fix inconsistency for NomBank
• fix random seeding in LanguageModel.generate
• fix ConditionalFreqDist mutation on tabulate/plot call
• fix broken links in documentation
• fix misc Wordnet issues
• update installation instructions

Thanks to the following contributors to 3.4.3: alvations, Bharat123rox, cifkao, drewmiller, free-variation, henchc irisxzhou, nick-ulle, ppartarr, simonepri, yigitsever, zhaoyanpeng

Commits

• 6404712 updates for 3.5
• 686ae7f updates for 3.5
• a12d395 Merge pull request #2530 from fievelk/fievelk/improve-stringtokenizer
• 24fd7b9 Improve StringTokenizer class
• d13078e Merge pull request #2515 from Hk-tang/remove-redundant-code
• c1bbe12 Removed unecessary keys call
• 821967a drop wininst as now deprecated: https://docs.python.org/3/distutils/builtdist...
• d22c840 include 3.8
• 5023d6b update copyright year
• cfa41ce updates for NLTK 3.5
• Additional commits viewable in compare view

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