Update go versions used in development (#2934)

Adding `.x` to Travis means use the latest minor version according to its docs.

I didn't update the "Requirements" header because this is just because of Go Modules; see #2932 for why. Since it's just a development change, it doesn't impact _using_ the provider.

Although, out of curiosity- is that header for development? Or is there a real requirement to have Go installed to use Terraform providers?

Author: rileykarson

.travis.yml

@@ -4,7 +4,7 @@ services:
 - docker
 language: go
 go:
-- "1.11"
+- "1.11.x"
 
 install:
 # This script is used by the Travis build to install a cookie for

README.md

@@ -63,7 +63,7 @@ $ make build
 Developing the provider
 ---------------------------
 
-If you wish to work on the provider, you'll first need [Go](http://www.golang.org) installed on your machine (version 1.11+ is *required*). You'll also need to correctly setup a [GOPATH](http://golang.org/doc/code.html#GOPATH), as well as adding `$GOPATH/bin` to your `$PATH`.
+If you wish to work on the provider, you'll first need [Go](http://www.golang.org) installed on your machine (version 1.11.4+ is *required*). You'll also need to correctly setup a [GOPATH](http://golang.org/doc/code.html#GOPATH), as well as adding `$GOPATH/bin` to your `$PATH`.
 
 To compile the provider, run `make build`. This will build the provider and put the provider binary in the `$GOPATH/bin` directory.