Min version req for skimage in tests

[s-rog]

What does this PR do?

Adds a minimum version for skimage in tests since old versions do not contain the needed metrics package

Trace (skimage 0.15.x)

================================================== ERRORS ==================================================
__________________________ ERROR collecting tests/metrics/regression/test_psnr.py __________________________
tests/metrics/regression/test_psnr.py:6: in <module>
    from skimage.metrics import peak_signal_noise_ratio
E   ModuleNotFoundError: No module named 'skimage.metrics'
__________________________ ERROR collecting tests/metrics/regression/test_psnr.py __________________________
ImportError while importing test module '/home/roger/pytorch-lightning-dev/tests/metrics/regression/test_psnr.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/opt/conda/lib/python3.6/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/metrics/regression/test_psnr.py:6: in <module>
    from skimage.metrics import peak_signal_noise_ratio
E   ModuleNotFoundError: No module named 'skimage.metrics'
__________________________ ERROR collecting tests/metrics/regression/test_ssim.py __________________________
tests/metrics/regression/test_ssim.py:6: in <module>
    from skimage.metrics import structural_similarity
E   ModuleNotFoundError: No module named 'skimage.metrics'
__________________________ ERROR collecting tests/metrics/regression/test_ssim.py __________________________
ImportError while importing test module '/home/roger/pytorch-lightning-dev/tests/metrics/regression/test_ssim.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/opt/conda/lib/python3.6/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/metrics/regression/test_ssim.py:6: in <module>
    from skimage.metrics import structural_similarity
E   ModuleNotFoundError: No module named 'skimage.metrics'

[codecov]

Codecov Report

Merging #4598 (3d1ae50) into master (30ad3e2) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #4598   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         116     116           
  Lines        8854    8854           
======================================
- Hits         8247    8244    -3     
- Misses        607     610    +3