Kubernetes 1.7 requires >= go1.8

Bump travis and bump darwin cross build container image

.travis.yml

@@ -4,7 +4,7 @@ sudo: require
 dist: trusty
 
 go:
-  - 1.8.1
+  - 1.8.3
 go_import_path: k8s.io/minikube
 
 install:

Makefile

@@ -21,7 +21,7 @@ DEB_VERSION ?= $(VERSION_MAJOR).$(VERSION_MINOR)-$(VERSION_BUILD)
 INSTALL_SIZE ?= $(shell du out/minikube-windows-amd64.exe | cut -f1)
 BUILDROOT_BRANCH ?= 2017.02
 REGISTRY?=gcr.io/k8s-minikube
-DARWIN_BUILD_IMAGE ?= karalabe/xgo-1.7.3
+DARWIN_BUILD_IMAGE ?= karalabe/xgo-1.8.3
 ISO_BUILD_IMAGE ?= $(REGISTRY)/buildroot-image
 
 # The iso will be versioned the same as minikube

docs/contributors/build_guide.md

@@ -1,5 +1,5 @@
 ### Build Requirements
-* A recent Go distribution (>1.7)
+* A recent Go distribution (>1.8)
 * If you're not on Linux, you'll need a Docker installation
 * Minikube requires at least 4GB of RAM to compile, which can be problematic when using docker-machine