Use 32 bits integer for repeat field to avoid overflow with frequent scheduled tasks

[gchardon-hiventy]

Simple field migration as discussed in #255

[codecov-io]

Codecov Report

Merging #262 into master will decrease coverage by 2.47%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
- Coverage    93.2%   90.72%   -2.48%     
==========================================
  Files          41       42       +1     
  Lines        2724     2729       +5     
==========================================
- Hits         2539     2476      -63     
- Misses        185      253      +68

Impacted Files	Coverage Δ
django_q/migrations/0009_auto_20171009_0915.py	100% <100%> (ø)
django_q/models.py	98.66% <100%> (ø)	⬆️
django_q/brokers/ironmq.py	0% <0%> (-94.74%)	⬇️
django_q/tests/test_brokers.py	70.58% <0%> (-11.77%)	⬇️
django_q/brokers/__init__.py	94.18% <0%> (-2.33%)	⬇️

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