Switch use of strings.LastIndexByte to strings.LastIndex.

It has almost zero impact on performance in practice,
and permits this code to keep working with Go 1.4.

Fixes #146.

protoc-gen-go/generator/generator.go

diff --git a/protoc-gen-go/generator/generator.go b/protoc-gen-go/generator/generator.go
index 2d431dc..2994cc9 100644
--- a/protoc-gen-go/generator/generator.go
+++ b/protoc-gen-go/generator/generator.go
@@ -275,7 +275,7 @@
 	}
 	ok = true
 	// The presence of a slash implies there's an import path.
-	slash := strings.LastIndexByte(pkg, '/')
+	slash := strings.LastIndex(pkg, "/")
 	if slash < 0 {
 		return
 	}