fixed Travis, converted README and version as file

[StrikerRUS]

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• [n/a] I have added necessary documentation about the functionality in the appropriate .md file
• [n/a] I have added in line documentation to the code I modified

Short description of what this PR does:

- fix tests and coverage by removing the check of existence of Dockerfile, docker-compose.yml, USE_CASES.md and using only codecov for coverage.

• add VERSION.txt file, so in the future version should be incremented only in this file.
• convert README.md to README.rst (appearance can be checked in this branch https://github.com/StrikerRUS/open-source-library-data-collector/blob/pypi/README.rst), so fix_README.py and register.py are not needed anymore.
• add Python versions badge.
  
• add PyPI version badge.
  

[SendGridDX]

All committers have signed the CLA.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@6dd1ae1). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #138   +/-   ##
=========================================
  Coverage          ?   48.71%           
=========================================
  Files             ?        2           
  Lines             ?      156           
  Branches          ?        0           
=========================================
  Hits              ?       76           
  Misses            ?       80           
  Partials          ?        0

Impacted Files	Coverage Δ
test/test_repository_files.py	100% <ø> (ø)

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[StrikerRUS]

Hi @thinkingserious !
You've asked me in sendgrid/python-http-client#93 (comment) to propose those changes in other SendGrid repos. This is the last one. Due to the README file replacement this PR has many conflicts with other open PRs. Is it possible to merge it first to avoid multiple branch rebases?

[StrikerRUS]

Gentle ping @thinkingserious

[StrikerRUS]

ping @thinkingserious

[StrikerRUS]

Closed due to the disinterest.