Change build settings to speed up builds

1. Don't run long steps for Go versions other than 1.9 & 1.10
according to Golang Release Policy (two latest versions).

2. Switch to codecov.io, collect coverage only on Go 1.10 which
has fixes for multi-module coverage & ./... ignoring vendor.

3. Simplify Makefile.

.gitignore

@@ -22,8 +22,7 @@ _testmain.go
 *.exe
 *.test
 
-coverage.html
-coverage*.out
+coverage.txt
 
 *.pyc