drop support for Go 1.13.x, test on 1.15.x

.github/workflows/test.yml

@@ -4,7 +4,7 @@ jobs:
   test:
     strategy:
       matrix:
-        go-version: [1.13.x, 1.14.x]
+        go-version: [1.14.x, 1.15.x]
         platform: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.platform }}
     steps:

README.md

@@ -2,7 +2,7 @@
 
 	GO111MODULE=on go get mvdan.cc/garble
 
-Obfuscate a Go build. Requires Go 1.13 or later.
+Obfuscate a Go build. Requires Go 1.14 or later.
 
 	garble build [build flags] [packages]
 
@@ -94,4 +94,4 @@ func init() { hideFatalErrors(true) }
 func main() {
 	panic("ya like jazz?")
 }
-```
+```