Maintain my own file list for hex

Apparently hex only includes some files by default but not others
rather than a sane default of anything except for VCS directories. So
now I also get to maintain a list of files in my .app.src.

src/jiffy.app.src

@@ -5,5 +5,17 @@
     {applications, [kernel, stdlib]},
     {maintainers, ["Paul J. Davis"]},
     {licenses, ["MIT", "BSD"]},
-    {links, [{"GitHub", "https://github.com/davisp/jiffy"}]}
+    {links, [{"GitHub", "https://github.com/davisp/jiffy"}]},
+    {files, [
+        "LICENSE",
+        "Makefile",
+        "README.md",
+        "c_src",
+        "enc",
+        "plugins",
+        "rebar.config",
+        "rebar.config.script",
+        "src",
+        "test"
+    ]}
 ]}.