feat: Adding golang 1.13 to the CI matrix (#294)

* Adding golang 1.13 to the CI matrix

* Drop Go 1.9 and 1.10

* We support Go v1.11 and higher.

Author: shogo82148

.travis.yml

@@ -1,10 +1,9 @@
 language: go
 
 go:
-  - "1.9.x"
-  - "1.10.x"
   - "1.11.x"
   - "1.12.x"
+  - "1.13.x"
   - master
 
 matrix:

README.md

@@ -41,9 +41,9 @@ requests, code review feedback, and also pull requests.
 
 ## Supported Go Versions
 
-We support Go v1.9 and higher.
+We support Go v1.11 and higher.
 [Continuous integration](https://travis-ci.org/firebase/firebase-admin-go) system
-tests the code on Go v1.9 through v1.10.
+tests the code on Go v1.11 through v1.13.
 
 ## Documentation