Bump To Go 1.15.5

The main k/k repo as updated to Go 1.15.5 for the upcoming k8s v1.20.0
release. See below PR for reference.

kubernetes/kubernetes#95776

Dockerfile

@@ -11,7 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-FROM golang:1.15.2
+FROM golang:1.15.5
 
 WORKDIR /go/src/sigs.k8s.io/descheduler
 COPY . .