fix efficientnet links

[dscarmo]

This just changes the EfficientNet URLs to make the automatic download work, fixing #148 .

[pep8speaks]

Hello @dscarmo! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file models/utils.py:

Line 306:80: E501 line too long (126 > 79 characters)
Line 307:80: E501 line too long (126 > 79 characters)
Line 308:80: E501 line too long (126 > 79 characters)
Line 309:80: E501 line too long (126 > 79 characters)
Line 310:80: E501 line too long (126 > 79 characters)
Line 311:80: E501 line too long (126 > 79 characters)
Line 312:80: E501 line too long (126 > 79 characters)
Line 313:80: E501 line too long (126 > 79 characters)