.travis.yml: Test with go v1.15.7

Currently in CI the tests are only run with go v1.12, this adds also go
v1.15.7.

Excludes certain variants for v1.15.

.travis.yml

@@ -7,6 +7,7 @@ services: docker
 
 go:
 - 1.12.12
+- 1.15.7
 
 notifications:
   on_success: never
@@ -31,6 +32,19 @@ matrix:
     env: TARGET=linux-amd64-grpcproxy
   - go: 1.12.12
     env: TARGET=linux-386-unit
+  exclude:
+    - go: 1.15.7
+      env: TARGET=linux-amd64-integration-1-cpu
+    - go: 1.15.7
+      env: TARGET=linux-amd64-integration-2-cpu
+    - go: 1.15.7
+      env: TARGET=linux-amd64-unit-4-cpu-race
+    - go: 1.15.7
+      env: TARGET=linux-amd64-coverage
+    - go: 1.15.7
+      env: TARGET=linux-amd64-fmt-unit-go-tip-2-cpu
+    - go: 1.15.7
+      env: TARGET=linux-386-unit-1-cpu
 
 before_install:
 - if [[ $TRAVIS_GO_VERSION == 1.* ]]; then docker pull gcr.io/etcd-development/etcd-test:go${TRAVIS_GO_VERSION}; fi
@@ -84,7 +98,7 @@ script:
             && GO_BUILD_FLAGS='-v' GOARCH=ppc64le ./build"
         ;;
       linux-amd64-grpcproxy)
-        sudo HOST_TMP_DIR=/tmp TEST_OPTS="PASSES='build grpcproxy'" make docker-test
+        sudo HOST_TMP_DIR=/tmp TEST_OPTS="PASSES='build grpcproxy'" GO_VERSION=${TRAVIS_GO_VERSION} make docker-test
         ;;
       linux-386-unit)
         docker run --rm \