tmp remove pre-clean before build for travis

Makefile

@@ -8,7 +8,7 @@ clean:
 	@find $(PKG) -name 'rex.a' -delete
 	@find $(PKG) -name 'rex' -type d -print0|xargs -0 rm -r
 
-build: clean
+build:
 	@go get -v ./...
 
 test: build