Don't build packages for pull requests

Since we don't want to build packages for pull requests (as they cannot
get archived as artifacts), we need a slightly different build command,
and moving it to a script was cleaner.

.travis.yml

@@ -9,19 +9,23 @@ osx_image: xcode8
 
 env:
   - GITHUB_TOKEN=a27573e27bea23fb05393ead69511b09e3b224be
+
 cache:
   directories:
     - $HOME/.dotnet
     - $HOME/.nuget
+
 addons:
   artifacts:
     s3_region: "us-west-2"
     paths: $(ls powershell*{deb,pkg} | tr "\n" ":")
+
 git:
   submodules: false
+
 install:
   - git config --global url.git@github.com:.insteadOf https://github.com/
   - git submodule update --init
   - (cd tools && ./download.sh)
-script:
-  - ulimit -n 4096; powershell -c "Import-Module ./build.psm1; Start-PSBootstrap -Package; Start-PSBuild -Publish; Start-PSPackage; Start-PSPester; Start-PSxUnit"
+
+script: ./tools/travis.sh