🚀 add support for singularity 2.6.1

[jsmedmar]

Singularity commands for 2.6 need to be slightly different

• 🚀   New feature
• 🔧   Code refactor
• 📘   I have updated the documentation accordingly
• ✅   I have added tests to cover my changes

[codecov]

Codecov Report

Merging #26 into master will decrease coverage by 0.73%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
- Coverage     100%   99.26%   -0.74%     
==========================================
  Files           5        5              
  Lines         267      272       +5     
==========================================
+ Hits          267      270       +3     
- Misses          0        2       +2

Impacted Files	Coverage Δ
toil_container/containers.py	98.03% <66.66%> (-1.97%)	⬇️

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