Fixes djangocms-picture compatibility issues and removed Django <=1.10 support

[FinalAngel]

• Removed support for Django 1.8, 1.9, 1.10
• Align migration typo fix with djangocms-picture
• Fix a regression where the bootstrap4_picture plugin creates an IntegrityError null value in column "use_responsive_image" violates not-null constraint
• Make sure migrations use proper variable values
• Also fixes #67

I'm not sure if abstract = False is required, but I assume it is best practice.

[codecov-io]

Codecov Report

Merging #68 into master will decrease coverage by 0.07%. The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
- Coverage   67.13%   67.05%   -0.08%     
==========================================
  Files          64       64              
  Lines        1287     1290       +3     
  Branches       95       95              
==========================================
+ Hits          864      865       +1     
- Misses        415      416       +1     
- Partials        8        9       +1

Impacted Files	Coverage Δ
...trib/bootstrap4_picture/migrations/0001_initial.py	100% <ø> (ø)	:arrow_up:
..._bootstrap4/contrib/bootstrap4_utilities/models.py	76% <0%> (ø)	:arrow_up:
...ms_bootstrap4/contrib/bootstrap4_picture/models.py	92.85% <100%> (+1.19%)	:arrow_up:
...ons/0002_bootstrap4picture_use_responsive_image.py	100% <100%> (ø)	:arrow_up:
...gocms_bootstrap4/contrib/bootstrap4_grid/models.py	80% <0%> (-2.5%)	:arrow_down:

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