fix travis.yml due to twill vs. pip
fix travis.yml due to twill vs. pip

adapted from 1f586ee

chadmiller/securedrop,ehartsuyker/securedrop,harlo/securedrop,ehartsuyker/securedrop,jrosco/securedrop,jaseg/securedrop,GabeIsman/securedrop,jrosco/securedrop,heartsucker/securedrop,pwplus/securedrop,ageis/securedrop,micahflee/securedrop,conorsch/securedrop,jaseg/securedrop,jeann2013/securedrop,garrettr/securedrop,GabeIsman/securedrop,heartsucker/securedrop,harlo/securedrop,pwplus/securedrop,jaseg/securedrop,harlo/securedrop,pwplus/securedrop,chadmiller/securedrop,conorsch/securedrop,garrettr/securedrop,conorsch/securedrop,jrosco/securedrop,kelcecil/securedrop,kelcecil/securedrop,micahflee/securedrop,harlo/securedrop,GabeIsman/securedrop,ageis/securedrop,garrettr/securedrop,ehartsuyker/securedrop,kelcecil/securedrop,harlo/securedrop,ehartsuyker/securedrop,harlo/securedrop,chadmiller/securedrop,jaseg/securedrop,jeann2013/securedrop,kelcecil/securedrop,chadmiller/securedrop,jrosco/securedrop,jeann2013/securedrop,heartsucker/securedrop,jaseg/securedrop,ageis/securedrop,conorsch/securedrop,heartsucker/securedrop,jeann2013/securedrop,conorsch/securedrop,jrosco/securedrop,GabeIsman/securedrop,pwplus/securedrop,garrettr/securedrop,kelcecil/securedrop,ageis/securedrop,jeann2013/securedrop,chadmiller/securedrop,micahflee/securedrop,kelcecil/securedrop,micahflee/securedrop,pwplus/securedrop,chadmiller/securedrop,heartsucker/securedrop,GabeIsman/securedrop,pwplus/securedrop,jrosco/securedrop,jaseg/securedrop,GabeIsman/securedrop,jeann2013/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop