Chocolatey updates the system PATH in the Registry, but it doesn’t get applied in any running applications. To reload it from the Registry you’ll need to use RefreshEnv.cmd (for a cmd.exe session) or RefreshEnv.ps1 (for a PowerShell session).
So, keeping in mind that your .travis.yml commands run inside a Git Bash shell, instead of iisexpress.exe you’ll need to run:
cmd.exe //c "RefreshEnv.cmd & iisexpress.exe"