switch to Go 1.15

Go 1.15 was released and is the major version that Kubernetes 1.19.0
is going to use. There are probably bugs in the older 1.13.3 that were
fixed, so we should update.

travis.yml

@@ -6,7 +6,7 @@ git:
   depth: false
 matrix:
   include:
-  - go: 1.13.3
+  - go: 1.15
 before_script:
 - mkdir -p bin
 - wget https://github.com/golang/dep/releases/download/v0.5.1/dep-linux-amd64 -O bin/dep