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.

tools/travis.sh

@@ -0,0 +1,7 @@
+ulimit -n 4096
+# Only build packages for branches, not pull requests
+if [[ $TRAVIS_PULL_REQUEST == false ]]; then
+    powershell -c "Import-Module ./build.psm1; Start-PSBootstrap -Package; Start-PSBuild -Publish; Start-PSPackage; Start-PSPester; Start-PSxUnit"
+else
+    powershell -c "Import-Module ./build.psm1; Start-PSBootstrap; Start-PSBuild; Start-PSPester; Start-PSxUnit"
+fi