Set -u to ensure PLAT is set

Treat unset variables as an error when substituting
to ensure that PLAT is set and not empty.

Author: bsolomon1124

travis/build-wheels.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-set -e -x
+set -e -u -x
 
 function repair_wheel {
     wheel="$1"