Feat/add codecov

[ciband]

Proposed changes

• Add Codecov support to Travis CI
• Add codecov npm dependency for development

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improve a current implementation without adding a new feature or fixing a bug)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build (changes that affect the build system)
• Docs (documentation only changes)
• Test (adding missing tests or fixing existing tests)
• Other... Please describe:

Checklist

• I have read the CONTRIBUTING documentation
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

Fixes #805

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@a2fe69c). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #821   +/-   ##
=========================================
  Coverage          ?   44.19%           
=========================================
  Files             ?      425           
  Lines             ?     6963           
  Branches          ?      883           
=========================================
  Hits              ?     3077           
  Misses            ?     3351           
  Partials          ?      535

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