internal: drop support for 1.9 and 1.10

As of Oct 1, 2019 GCP is no longer supporting Go 1.9 and 1.10.
Cleaning up build scripts to support this.

Change-Id: I48a939fd7918c52cce5994a2746f215e3fbe1750
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/46050
Reviewed-by: Jean de Klerk <deklerk@google.com>
Reviewed-by: kokoro <noreply+kokoro@google.com>
diff --git a/internal/kokoro/continuous.sh b/internal/kokoro/continuous.sh
index 814da64..0c5629d 100755
--- a/internal/kokoro/continuous.sh
+++ b/internal/kokoro/continuous.sh
@@ -40,6 +40,8 @@
 export GOPATH="$HOME/go"
 export GOCLOUD_HOME=$GOPATH/src/cloud.google.com/go/
 export PATH="$GOPATH/bin:$PATH"
+export GO111MODULE=on
+
 
 # Move code into $GOPATH and get dependencies
 mkdir -p $GOCLOUD_HOME
@@ -48,21 +50,8 @@
 
 try3() { eval "$*" || eval "$*" || eval "$*"; }
 
-download_deps() {
-    if [[ `go version` == *"go1.11"* ]] || [[ `go version` == *"go1.12"* ]] || [[ `go version` == *"go1.13"* ]]; then
-        export GO111MODULE=on
-        # All packages, including +build tools, are fetched.
-        try3 go mod download
-    else
-        # Because we don't provide -tags tools, the +build tools
-        # dependencies aren't fetched.
-        try3 go get -v -t ./...
-
-        go get github.com/jstemmer/go-junit-report
-    fi
-}
-
-download_deps
+# All packages, including +build tools, are fetched.
+try3 go mod download
 go install github.com/jstemmer/go-junit-report
 ./internal/kokoro/vet.sh
 
diff --git a/internal/kokoro/presubmit.sh b/internal/kokoro/presubmit.sh
index 06723ef..d396f24 100755
--- a/internal/kokoro/presubmit.sh
+++ b/internal/kokoro/presubmit.sh
@@ -30,6 +30,7 @@
 export GOPATH="$HOME/go"
 export GOCLOUD_HOME=$GOPATH/src/cloud.google.com/go/
 export PATH="$GOPATH/bin:$PATH"
+export GO111MODULE=on
 
 # Move code into $GOPATH and get dependencies
 mkdir -p $GOCLOUD_HOME
@@ -38,21 +39,8 @@
 
 try3() { eval "$*" || eval "$*" || eval "$*"; }
 
-download_deps() {
-    if [[ `go version` == *"go1.11"* ]] || [[ `go version` == *"go1.12"* ]] || [[ `go version` == *"go1.13"* ]]; then
-        export GO111MODULE=on
-        # All packages, including +build tools, are fetched.
-        try3 go mod download
-    else
-        # Because we don't provide -tags tools, the +build tools
-        # dependencies aren't fetched.
-        try3 go get -v -t ./...
-
-        go get github.com/jstemmer/go-junit-report
-    fi
-}
-
-download_deps
+# All packages, including +build tools, are fetched.
+try3 go mod download
 go install github.com/jstemmer/go-junit-report
 ./internal/kokoro/vet.sh
 ./internal/kokoro/check_incompat_changes.sh