Needs to set documenter key

[johnnychen94]

The last time gh-pages got updated is two years ago...

┌ Info: Deployment criteria for deploying devbranch build from Travis:
│ - ✔ ENV["TRAVIS_REPO_SLUG"]="JuliaImages/ImageAxes.jl" occurs in repo="github.com/JuliaImages/ImageAxes.jl.git"
│ - ✔ ENV["TRAVIS_PULL_REQUEST"]="false" is "false"
│ - ✔ ENV["TRAVIS_BRANCH"] matches devbranch="master"
│ - ✘ ENV["DOCUMENTER_KEY"] exists
│ - ✔ ENV["TRAVIS_EVENT_TYPE"]="push" is not "cron"
└ Deploying: ✘

Perhaps we also need to update docs as well (e.g., add a Project.toml)