Reverts Go to v1.17 (from v1.18)

The travis build was failing with something like this:

mattn/goveralls#207

... and the Docker build was failing with this error:

Step 8/10 : COPY --from=build /go/bin/pusher /
COPY failed: stat go/bin/pusher: file does not exist

Author: nkinkade

.travis.yml

@@ -1,7 +1,7 @@
 language: go
 
 go:
-- "1.18"
+- "1.17"
 
 services:
 - docker

Dockerfile

@@ -1,4 +1,4 @@
-FROM golang:1.18 as build
+FROM golang:1.17 as build
 # Add the local files to be sure we are building the local source code instead
 # of downloading from GitHub.
 # Don't add any of the other libraries, because we live at HEAD.