Getting development snapshots working with travis ci

i’m trying to use the compiler development snapshots with the Travis CI service on MacOS, but the build is failing with this error:

error: manifest parse error(s):
<unknown>:0: error: Swift does not support the SDK 'MacOSX10.14.sdk'

i’m aware this has something to do with needing the latest xcode beta on the vm, but xcode 11 isn’t available on Travis. has anyone found a solution to this?

this is my travis.yml

language: generic
matrix:
  include:
  - os: osx
    osx_image: xcode10.2
    env:
    - JOB="MacOS"
  - os: linux
    env:
    - JOB="Linux"
    dist: trusty
script:
- export SWIFT_VERSION=DEVELOPMENT-SNAPSHOT-2019-07-01-a
- git clone --depth 1 https://github.com/kylef/swiftenv.git ~/.swiftenv
- export SWIFTENV_ROOT="$HOME/.swiftenv"
- export PATH="$SWIFTENV_ROOT/bin:$SWIFTENV_ROOT/shims:$PATH"
- swiftenv install $SWIFT_VERSION
- swiftenv rehash
- swift run -c release tests