Update requirements.txt (#271)

* Update requirements.txt

`timm` version has changed to 0.2.1 two months ago. Is it possible it can change here as well?

* Modified Activation Function Input

* Update timm_efficientnet.py

Author: ssakhavi

requirements.txt

@@ -1,4 +1,4 @@
 torchvision>=0.3.0
 pretrainedmodels==0.7.4
 efficientnet-pytorch==0.6.3
-timm==0.1.20
+timm==0.2.1

segmentation_models_pytorch/encoders/timm_efficientnet.py

@@ -1,8 +1,9 @@
 import torch
 import torch.nn as nn
 
-from timm.models.efficientnet import EfficientNet, Swish
+from timm.models.efficientnet import EfficientNet
 from timm.models.efficientnet import decode_arch_def, round_channels, default_cfgs
+from timm.models.layers.activations import Swish
 
 from ._base import EncoderMixin