unset GOBIN in Makefile

If the GOBIN environment variable is set to a non-default location
relative to GOPATH, then promu will be built in this location and the
the build will fail because it tries to run $GOPATH/bin/promu.

See also issue #483 and #492

Makefile

@@ -11,6 +11,9 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+# Ensure GOBIN is not set during build so that promu is installed to the correct path
+unexport GOBIN
+
 GO           ?= go
 GOFMT        ?= $(GO)fmt
 FIRST_GOPATH := $(firstword $(subst :, ,$(shell $(GO) env GOPATH)))