build both ubuntu xenial as well as trusty to get more language level
coverage, force C++98 in one of the C++ builds to guarantee compat,
and update docker readme

This closes #1377
diff --git a/lib/go/Makefile.am b/lib/go/Makefile.am
index f1bd0e6..0d59710 100644
--- a/lib/go/Makefile.am
+++ b/lib/go/Makefile.am
@@ -34,6 +34,9 @@
 	GOPATH=`pwd` $(GO) get golang.org/x/net/context
 	GOPATH=`pwd` $(GO) test -race ./thrift
 
+clean-local:
+	$(RM) -rf pkg
+
 all-local:
 	GOPATH=`pwd` $(GO) get golang.org/x/net/context
 	GOPATH=`pwd` $(GO) build ./thrift