Bump minimum rkt version to 1.27.0.

The changes introduces in #3256 require at least rkt 1.27.0 because of
a bug in the JSON output of `rkt status` in previous versions.

Here we upgrade all references to rkt's minimum version, and also make
travis and vagrant use this version when running tests.

Finally we add a CHANGELOG notice.

CHANGELOG.md

@@ -1,5 +1,10 @@
 ## 0.7 (Unreleased)
 
+__BACKWARDS INCOMPATIBILITIES:__
+ * driver/rkt: Nomad now requires at least rkt version `1.27.0` for the rkt
+   driver to function. Please update your version of rkt to at least this
+   version.
+
 IMPROVEMENTS:
  * core: Capability based ACL system with authoratative region, providing
    federated ACLs.

client/driver/rkt.go

@@ -44,7 +44,7 @@ const (
 	// minRktVersion is the earliest supported version of rkt. rkt added support
 	// for CPU and memory isolators in 0.14.0. We cannot support an earlier
 	// version to maintain an uniform interface across all drivers
-	minRktVersion = "1.0.0"
+	minRktVersion = "1.27.0"
 
 	// The key populated in the Node Attributes to indicate the presence of the
 	// Rkt driver

scripts/travis-rkt.sh

@@ -2,7 +2,7 @@
 
 set -o errexit
 
-VERSION=1.18.0
+VERSION=1.27.0
 DOWNLOAD=https://github.com/coreos/rkt/releases/download/v${VERSION}/rkt-v${VERSION}.tar.gz
 
 function install_rkt() {

scripts/vagrant-linux-priv-rkt.sh

@@ -2,7 +2,7 @@
 
 set -o errexit
 
-VERSION=1.18.0
+VERSION=1.27.0
 DOWNLOAD=https://github.com/coreos/rkt/releases/download/v${VERSION}/rkt-v${VERSION}.tar.gz
 
 function install_rkt() {

website/source/docs/drivers/rkt.html.md

@@ -180,8 +180,8 @@ The `rkt` driver will set the following client attributes:
 
 * `driver.rkt` - Set to `1` if rkt is found on the host node. Nomad determines
   this by executing `rkt version` on the host and parsing the output
-* `driver.rkt.version` - Version of `rkt` e.g.: `1.1.0`. Note that the minimum required
-  version is `1.0.0`
+* `driver.rkt.version` - Version of `rkt` e.g.: `1.27.0`. Note that the minimum required
+  version is `1.27.0`
 * `driver.rkt.appc.version` - Version of `appc` that `rkt` is using e.g.: `1.1.0`
 
 Here is an example of using these properties in a job file: