Fix broken tests (mapbox#96)

One of the transitive dependencies (protocol-buffers-schema) got
upgraded, causing some of the compile tests to break, most notably
with this message:

    Fields of type MessageType cannot be declared [packed=true]. Only
    repeated fields of primitive numeric types (types which use the
    varint, 32-bit, or 64-bit wire types) can be declared "packed". See
    https://developers.google.com/protocol-buffers/docs/encoding#optional

Remove the MessageType enum and use int32 in its place.

The new version of protocol-buffers-schema also changed how enums are
represented in json, breaking another test.

test/compile.test.js

@@ -126,7 +126,10 @@ test('compiles defaults', function(t) {
 
     t.equals(buf.length, 0);
     t.deepEqual(data, {
-        type: 1,
+        type: {
+            options: {},
+            value: 1
+        },
         name: 'test',
         flag: true,
         weight: 1.5,

test/fixtures/packed.proto

@@ -4,20 +4,15 @@ package vector_tile;
 
 option optimize_for = LITE_RUNTIME;
 
-enum MessageType {
-     UNKNOWN = 0;
-     GREETING = 1;
-}
-
 message NotPacked {
   repeated int32 value = 1;
-  repeated MessageType types = 2;
+  repeated int32 types = 2;
 }
 message FalsePacked {
   repeated int32 value = 1 [packed=false];
-  repeated MessageType types = 2 [packed=false];
+  repeated int32 types = 2 [packed=false];
 }
 message Packed {
   repeated int32 value = 1 [packed=true];
-  repeated MessageType types = 2 [packed=true];
+  repeated int32 types = 2 [packed=true];
 }