Add Go 1.13 to Travis (#165)

Signed-off-by: Gábor Lipták <gliptak@gmail.com>

.travis.yml

@@ -9,6 +9,7 @@ go:
   - "1.10"
   - "1.11"
   - "1.12"
+  - "1.13"
   - "tip"
 
 notifications:
@@ -39,18 +40,18 @@ install:
   - if [ $DEP_MANAGEMENT_TOOL == "DEP" ]; then
       dep ensure;
     else
-      go get;
+      go mod download;
     fi
   # Only bother running lints on the latest version.  Some of the linters fail to install on older versions of go.
-  - if [ $TRAVIS_GO_VERSION == "1.12" ]; then
+  - if [ $TRAVIS_GO_VERSION == "1.13" ]; then
       go get github.com/golangci/golangci-lint/cmd/golangci-lint@v1.18.0;
     fi
 
 script:
   - make test-with-race
   # We are in the process of resolving all the lint warnings for x-ray-sdk-go.  Still run the linter to expose these
   # and resolve to clean these up.
-  - if [ $TRAVIS_GO_VERSION == "1.12" ]; then
+  - if [ $TRAVIS_GO_VERSION == "1.13" ]; then
       make golangci-lint || true;
     fi