Set minimum version for NumPy requirement

[fepegar]

Fixes #529.

Description
Setting the NumPy requirement to numpy>=1.15.0 so an iterable for axes can be passed to np.flip.

Checklist

• I have read the CONTRIBUTING docs and have a developer setup (especially important are pre-commitand pytest)
• Non-breaking change (would not break existing functionality)
• Breaking change (would cause existing functionality to change)
• Tests added or modified to cover the changes
• Integration tests passed locally by running pytest
• In-line docstrings updated
• Documentation updated, tested running make html inside the docs/ folder
• This pull request is ready to be reviewed
• If the PR is ready and there are multiple commits, I have squashed them and force-pushed

[codecov]

Codecov Report

Merging #532 (5c53b2c) into master (c7af603) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #532   +/-   ##
=======================================
  Coverage   91.64%   91.64%           
=======================================
  Files         124      124           
  Lines        6521     6521           
=======================================
  Hits         5976     5976           
  Misses        545      545           

Impacted Files	Coverage Δ
...hio/transforms/augmentation/spatial/random_flip.py	100.00% <100.00%> (ø)

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