vet w/ go 1.13; use normal go proxy instead of direct in CI

.travis.yml

@@ -3,13 +3,10 @@ sudo: false
 
 matrix:
   include:
-    # TODO: update the version used to vet to Go 1.13
     - go: 1.11.x
       env: GO111MODULE=off
     - go: 1.11.x
-      env:
-      - GO111MODULE=on
-      - VET=1
+      env: GO111MODULE=on
     - go: 1.12.x
       env: GO111MODULE=off
     - go: 1.12.x
@@ -17,15 +14,11 @@ matrix:
     - go: 1.13.x
       env:
       - GO111MODULE=on
-      - GOPROXY=direct
+      - VET=1
     - go: 1.14.x
-      env:
-      - GO111MODULE=on
-      - GOPROXY=direct
+      env: GO111MODULE=on
     - go: tip
-      env:
-      - GO111MODULE=on
-      - GOPROXY=direct
+      env: GO111MODULE=on
 
 script:
   - if [[ "$VET" = 1 ]]; then make ci; else make deps test; fi