codec: update travis to run 3 sets of tests (unsafe, safe and codecgen)

This should give us a better idea that we work across the board.

.travis.yml

@@ -10,5 +10,7 @@ go:
     - tip
 script:
     - go test -tags "alltests" -run Suite -coverprofile coverage.txt github.com/ugorji/go/codec
+    - go test -tags "alltests safe" -run Suite -coverprofile coverage.safe.txt github.com/ugorji/go/codec
+    - go test -tags "alltests codecgen" -run Suite -coverprofile coverage.codecgen.txt github.com/ugorji/go/codec
 after_success:
     - bash <(curl -s https://codecov.io/bash)