Add travis-ci shield to manifest

Fixes #74
See #76
diff --git a/Cargo.toml b/Cargo.toml
index f8139ba..92a9f70 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -26,3 +26,7 @@
 
 [profile.bench]
 opt-level = 3
+
+[badges]
+
+travis-ci = { repository = "https://github.com/BurntSushi/byteorder", branch = "master" }