Simplify travis.yml with tox environment variables
Simplify travis.yml with tox environment variables

tareqalayan/pytest,jaraco/pytest,tomviner/pytest,flub/pytest,skylarjhdownes/pytest,nicoddemus/pytest,nicoddemus/pytest,pfctdayelise/pytest,pytest-dev/pytest,ddboline/pytest,rmfitzpatrick/pytest,alfredodeza/pytest,txomon/pytest,vmalloc/dessert,Akasurde/pytest,hackebrot/pytest,davidszotten/pytest,The-Compiler/pytest,The-Compiler/pytest,etataurov/pytest,tgoodlet/pytest,MichaelAquilina/pytest,tomviner/pytest,markshao/pytest,RonnyPfannschmidt/pytest