Replace references to bazel-genfiles with bazel-bin

In bazel 0.25 the target directory was changed from bazel-genfiles
to bazel-bin and bazel-genfiles was symlinked to bazel-bin.

In bazel 1.0 the symlink no longer gets created, and this causes the
./tools/maven/ script to fail on a clean workspace because it
still refers to the old location.

Fix to use bazel-bin.

Change-Id: I3ab9979d7f9345393592514c58e7ba06396e2491
diff --git a/tools/maven/ b/tools/maven/
index 93bef01..6fd6341 100755
--- a/tools/maven/
+++ b/tools/maven/
@@ -53,4 +53,4 @@
   { echo "bazel failed to build gen_${command}. Use VERBOSE=1 for more info" ; exit 1 ; }
 export OUTPUT_BASE=`bazel info output_base`