fix crash and support CodeBuild under CodePipeline

[klausbadelt]

CodeBuild under CodePipeline sets environment variables differently, causing Formatter SimpleCov::Formatter::Codecov failed with NoMethodError: undefined methodsplit' for nil:NilClass`. This PR fixes that.

Branch name and slug are supported too, but need some setup. These instructions are included in the source. (I couldn't a home for these elsewhere):

CodePipeline with CodeBuild

To setup branch and slug with CodePipeline as CodeBuild source:

1. Set up CodeStarSourceConnection as source action provider See AWS CodePipeline CodeStarConnection Reference.
2. Add a Namespace to your source action. Example: "CodeStar" See AWS CodePipeline Namespaces Reference.
3. Add these environment variables to your CodeBuild action:
   • CODESTAR_BRANCH_NAME: set to #{GitHub.BranchName}
   • CODESTAR_FULL_REPOSITORY_NAME: set to #{GitHub.FullRepositoryName} (optional) See AWS CodePipeline Action Reference.

(Uploading PR number support is unfortunately not included nor easily possible IMHO).

[codecov[bot]]

Codecov Report

Merging #131 (8b7b0ef) into master (e0ff57f) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #131      +/-   ##
==========================================
+ Coverage   91.84%   91.86%   +0.02%     
==========================================
  Files           3        3              
  Lines         368      369       +1     
==========================================
+ Hits          338      339       +1     
  Misses         30       30              

Impacted Files	Coverage Δ
lib/codecov/uploader.rb	91.29% <100.00%> (+0.02%)	:arrow_up:

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