Properly set the binary name in the promu config instead of doing
renaming acrobatics in the ebuild.

--- a/.promu.yml
+++ b/.promu.yml
@@ -5,6 +5,8 @@ go:
 repository:
   path: github.com/superq/chrony_exporter
 build:
+  binaries:
+    - name: chrony_exporter
   ldflags: |
     -X github.com/prometheus/common/version.Version={{.Version}}
     -X github.com/prometheus/common/version.Revision={{.Revision}}