(maint) - Resolving bundler ruby version failure, updating tests to include puppet 6

[pmcmaw]

Removing the pin from bundler as the older version of bundler does not support newer version of ruby. Also updating testing to include puppet 6.

This PR is required to allow #270 to progress.

[codecov-io]

Codecov Report

Merging #271 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #271      +/-   ##
==========================================
+ Coverage   44.37%   44.38%   +0.01%     
==========================================
  Files          11       11              
  Lines         773      775       +2     
==========================================
+ Hits          343      344       +1     
- Misses        430      431       +1

Impacted Files	Coverage Δ
...puppetlabs_spec_helper/puppetlabs_spec/matchers.rb	20.83% <0%> (-0.45%)	⬇️
lib/puppetlabs_spec_helper/tasks/check_symlinks.rb	100% <0%> (ø)	⬆️

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[david22swan]

Look's good to me, but you should probably get the opinion of someone who has worked more with the module.
+1