Split the SwiftPM Lib CI in two.

Looks like travis was taking a while for this as one large build, so
split it so the CI finishes a bit faster.

Author: thomasvl

.travis.yml

@@ -14,7 +14,8 @@ env:
   - TEST_TYPE=Xcode MODE=Example_DriveSample CFG=Both
   - TEST_TYPE=Xcode MODE=Example_YouTubeSample CFG=Both
   - TEST_TYPE=Xcode MODE=Example_StorageSample CFG=Both
-  - TEST_TYPE=SwiftPM MODE=Lib CFG=Both
+  - TEST_TYPE=SwiftPM MODE=Lib CFG=Debug
+  - TEST_TYPE=SwiftPM MODE=Lib CFG=Release
   - TEST_TYPE=SwiftPM MODE=ServiceGenerator CFG=Both
   - TEST_TYPE=Pod MODE=ios CFG=Debug
   - TEST_TYPE=Pod MODE=ios CFG=Release