bump kubernetes for cve-2019-11253 (#809)

Makefile

@@ -19,7 +19,7 @@ ASSETSDIR=$(TOP)/assets
 BINDIR ?= /usr/bin
 
 # Current Kubernetes version
-K8S_VER := 1.15.4
+K8S_VER := 1.15.5
 # Kubernetes version suffix for the planet package, constructed by concatenating
 # major + minor padded to 2 chars with 0 + patch also padded to 2 chars, e.g.
 # 1.13.5 -> 11305, 1.13.12 -> 11312, 2.0.0 -> 20000 and so on
@@ -42,7 +42,7 @@ RELEASE_OUT ?=
 TELEPORT_TAG = 3.2.7
 # TELEPORT_REPOTAG adapts TELEPORT_TAG to the teleport tagging scheme
 TELEPORT_REPOTAG := v$(TELEPORT_TAG)
-PLANET_TAG := 6.1.7-$(K8S_VER_SUFFIX)
+PLANET_TAG := 6.1.8-$(K8S_VER_SUFFIX)
 PLANET_BRANCH := $(PLANET_TAG)
 K8S_APP_TAG := $(GRAVITY_TAG)
 TELEKUBE_APP_TAG := $(GRAVITY_TAG)