Update Travis script to install gradients plug-ins
Update Travis script to install gradients plug-ins

bygreencn/DIGITS,gheinrich/DIGITS-GAN,ethantang95/DIGITS-GAN,gheinrich/DIGITS-GAN,Deepomatic/DIGITS,Deepomatic/DIGITS,gheinrich/DIGITS-GAN,ethantang95/DIGITS,TimZaman/DIGITS,Deepomatic/DIGITS,ethantang95/DIGITS-GAN,ethantang95/DIGITS-GAN,TimZaman/DIGITS,gheinrich/DIGITS-GAN,Deepomatic/DIGITS,TimZaman/DIGITS,bygreencn/DIGITS,TimZaman/DIGITS,ethantang95/DIGITS,ethantang95/DIGITS-GAN,ethantang95/DIGITS,ethantang95/DIGITS,bygreencn/DIGITS,bygreencn/DIGITS