Drop Py2.7 and add Korean and Hebrew languages

What's Changed

• INCOMPATIBILITY Dropped official support for Python 2.7. It should still work if you install Python 2 compatible dependencies.
• FEATURE: Add basic Korean support by @kimbyungnam in #129
• FEATURE: Add support for the Hebrew language by @miso-belica in #151
• FIX: Allow words with dashes/apostrophe returned from tokenizer by @miso-belica in #144
• FIX: Ignore empty sentences from tokenizer by @miso-belica in #153
• Basic documentation by @miso-belica in #133
• Speedup of the TextRank algorithm by @miso-belica in #140
• Fix missing license in sdist by @dopplershift in #157
• added test and call for stemmer by @bdalal in #131
• Fix simple typo: referene -> reference by @timgates42 in #143
• Add codecov service to tests by @miso-belica in #136
• Add gitpod config by @miso-belica in #138
• Try to run Python 3.7 and 3.8 on TravisCI by @miso-belica in #130
• Fix TravisCI for Python 3.4 by @miso-belica in #134

New Contributors

• @bdalal made their first contribution in #131
• @kimbyungnam made their first contribution in #129
• @timgates42 made their first contribution in #143
• @dopplershift made their first contribution in #157

Full Changelog: v0.8.1...v0.8.2