Merge pull request kubernetes-csi#97 from pohly/go-1.15

switch to Go 1.15

Author: k8s-ci-robot

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