all: update go.mod files to target Go 1.13

Since the main README recommends Go 1.13 or later, let's make the go.mod
files reflect that. This will enable starting to use new language
features.

Modules that still build on 1.12 will continue to work on that version
just fine; this line is just a hint to enable new language features for
versions of Go new enough.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

apps/go.mod

@@ -1,6 +1,6 @@
 module gioui.org/apps
 
-go 1.12
+go 1.13
 
 require (
 	gioui.org/ui v0.0.0-20190902131729-7ad60e08787b

cmd/go.mod

@@ -1,6 +1,6 @@
 module gioui.org/cmd
 
-go 1.12
+go 1.13
 
 require (
 	golang.org/x/image v0.0.0-20190802002840-cff245a6509b

ui/go.mod

@@ -1,6 +1,6 @@
 module gioui.org/ui
 
-go 1.12
+go 1.13
 
 require (
 	golang.org/x/image v0.0.0-20190703141733-d6a02ce849c9

website/go.mod

@@ -1,3 +1,3 @@
 module gioui.org/website
 
-go 1.12
+go 1.13