internal/proxy: port script from Go Cloud to create module proxy (#95)

I have created gs://wire-modules to store the Wire dependencies. I ran
the script in this commit to populate the contents of the GCS bucket and
am updating Travis to point to the new proxy.

Fixes #81

.travis.yml

@@ -45,5 +45,5 @@ script:
 
 env:
   global:
-  - GOPROXY=https://storage.googleapis.com/go-cloud-modules/
+  - GOPROXY=https://storage.googleapis.com/wire-modules/
   - GO111MODULE=on