travis: add go1.13 and remove go1.10 and go1.9 without GAE (#3050)

diff --git a/.travis.yml b/.travis.yml
index f0f723f..c6762c2 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -2,18 +2,18 @@
 
 matrix:
   include:
-  - go: 1.12.x
+  - go: 1.13.x
     env: VET=1 GO111MODULE=on
-  - go: 1.12.x
+  - go: 1.13.x
     env: RACE=1 GO111MODULE=on
-  - go: 1.12.x
+  - go: 1.13.x
     env: RUN386=1
-  - go: 1.12.x
+  - go: 1.13.x
     env: GRPC_GO_RETRY=on
+  - go: 1.12.x
+    env: GO111MODULE=on
   - go: 1.11.x
     env: GO111MODULE=on
-  - go: 1.10.x
-  - go: 1.9.x
   - go: 1.9.x
     env: GAE=1